Can a retail trader do bid-ask spread scalping through algo-trading?
"In US public stock markets there is no difference between the actions individual retail traders are ""permitted"" to take and the actions institutional/corporate traders are ""permitted"" to take.  The only difference is the cost of those actions.  For example, if you become a Registered Market Maker on, say, the BATS stock exchange, you'll get some amazing rebates and reduced transaction prices; however, in order to qualify for Registered Market Maker status you have to maintain constant orders in the book for hundreds of equities at significant volumes.  An individual retail trader is certainly permitted to do that, but it's probably too expensive. Algorithmic trading is not the same as automated trading (algorithmic trading can be non-automated, and automated trading can be non-algorithmic), and both can be anywhere from low- to high-frequency.  A low-frequency automated strategy is essentially indistinguishable from a person clicking their mouse several times per day, so: no, from a legal or regulatory perspective there is no special procedure an individual retail trader has to follow before s/he can automate a trading strategy.  (Your broker, on the other hand, may have all sorts of hoops for you to jump through in order to use their automation platform.) Last (but certainly not least) you will almost certainly lose money hand over fist attempting bid-ask scalping as an individual retail trader, whether your approach is algorithmic or not, automated or not.  Why?  Because the only way to succeed at bid-ask scalping is to (a) always be at/near the front of the queue when a price change occurs in your favor, and (b) always cancel your resting orders before they are executed when a price change occurs against you.  Unless your algorithms are smarter than every other algorithm in the industry, an individual retail trader operating through a broker's trading platform cannot react quickly enough to succeed at either of those.  You would have to eschew the broker and buy direct market access to even have a chance, and that's the point at which you're no longer a retail trader. Good luck!"
I disagree strongly with the other two answers posted thus far. HFT are not just liquidity providers (in fact that claim is completely bogus, considering liquidity evaporates whenever the market is falling). HFT are not just scalping for pennies, they are also trading based on trends and news releases. So you end up having imperfect algorithms, not humans, deciding the price of almost every security being traded. These algorithms data mine for news releases or they look for and make correlations, even when none exist.  The result is that every asset traded using HFT is mispriced. This happens in a variety of ways. Algos will react to the same news event if it has multiple sources (Ive seen stocks soar when week old news was re-released), algos will react to fake news posted on Twitter, and algos will correlate S&P to other indexes such as VIX or currencies. About 2 years ago the S&P was strongly correlated with EURJPY. In other words, the American stock market was completely dependent on the exchange rate of two currencies on completely different continents. In other words, no one knows the true value of stocks anymore because the free market hasnt existed in over 5 years. "In general, there should be a ""liquidity premium"" which means that less-liquid stocks should be cheaper. That's because to buy such a stock, you should demand a higher rate of return to compensate for the liquidity risk (the possibility that you won't be able to sell easily). Lower initial price = higher eventual rate of return. That's what's meant when Investopedia says the security would be cheaper (on average). Is liquidity good? It depends. Here's what illiquidity is. Imagine you own a rare piece of art. Say there are 10 people in the world who collect this type of art, and would appreciate what you own. That's an illiquid asset, because when you want to sell, maybe those 10 people aren't buying - maybe they don't want your particular piece, or they all happen to be short on funds. Or maybe worse, only one of them is buying, so they have all the negotiating leverage. You'll have to lower your price if you're really in a hurry to sell. Maybe if you lower your price enough, you can get one of the 10 buyers interested, even if none were initially. An illiquid asset is bad for sellers. Illiquid means there aren't enough buyers for you to get a bidding war going at the time of your choosing. You'll potentially have to wait around for buyers to turn up, or for a stock, maybe you'd have to sell a little bit at a time as buyers want the shares. Illiquid can be bad for buyers, too, if the buyer is for some reason in a hurry; maybe nobody is selling at any given time. But, usually buyers don't have to be in a hurry. An exception may be if you short sell something illiquid (brokers often won't let you do this, btw). In that case you could be a forced buyer and this could be very bad on an illiquid security. If there are only one or two sellers out there, they now have the negotiating leverage and they can ask whatever price they want. Illiquidity is very bad when mixed with margin or short sales because of the potential for forced trades at inopportune times. There are plenty of obscure penny stocks where there might be only one or two trades per day, or fewer. The spread is going to be high on these because the bids at a given time will just be lowball offers from buyers who aren't really all that interested, unless you want to give your stock away, in which case they'll take it. And the asks are going to be from sellers who want to get a decent price, but maybe there aren't really any buyers willing to pay, so the ask is just sitting there with no takers. The bids and asks may be limit orders that have been sitting open for 3 weeks and forgotten about. Contrast with a liquid asset. For example, a popular-model used car in good condition would be a lot more liquid than a rare piece of art, though not nearly as liquid as most stocks. You can probably find several people that want to buy it living nearby, and you're not going to have to drop the price to get a buyer to show up. You might even get those buyers in a bidding war. From illiquid penny stocks, there's a continuum all the way up to the most heavily-traded stocks such as those in the S&P500. With these at a given moment there will be thousands of buyers and sellers, so the spread is going to close down to nearly zero. If you think about it, just statistically, if there are thousands of bids and thousands of asks, then the closest bid-ask pair is going to be close together. That's a narrow spread. While if there are 3 bids and 2 asks on some illiquid penny stock, they might be dollars away from each other, and the number of shares desired might not match up. You can see how liquidity is good in some situations and not in others. An illiquid asset gives you more opportunity to get a good deal because there aren't a lot of other buyers and sellers around and there's some opportunity to ""negotiate"" within the wide spread. For some assets maybe you can literally negotiate by talking to the other party, though obviously not when trading stocks on an exchange. But an illiquid asset also means you might get a bad deal, especially if you need to sell quickly and the only buyers around are making lowball offers. So the time to buy illiquid assets is when you can take your time on both buying and selling, and will have no reason for a forced trade on a particular timeline. This usually means no debt is involved, since creditors (including your margin broker) can force you to trade. It also means you don't need to spend the money anytime soon, since if you suddenly needed the money you'd have a forced trade on your hands. If you have the time, then you put a price out there that's very good for you, and you wait for someone to show up and give you that price - this is how you get a good deal. One more note, another use of the term liquid is to refer to assets with low or zero volatility, such as money market funds. An asset with a lot of volatility around its intrinsic or true value is effectively illiquid even if there's high trade volume, in that any given point in time might not be a good time to sell, because the price isn't at the right level. Anyway, the general definition of a liquid investment is one that you'd be comfortable cashing out of at a moment's notice. In this sense, most stocks are not all that liquid, despite high trading volume. In different contexts people may use ""liquid"" in this sense or to mean a low bid-ask spread." While open interest usually correlates to volume, the mark of liquidity is the bid ask spread. Even when trading options with spreads as large as an ask 2x the bid, a more realistic price that traders are willing to accept lies somewhere in the middle. Any option can easily be exited at intrinsic value: underlying price - exercise price for calls, exercise price - underlying price for puts.  For illiquid options, this will be the best price obtained.  For longer term options, something closer to the theoretical price is still possible. If an underlying is extremely liquid, yet the options aren't quite then options traders will be much more ready to trade at the theoretical price. For exiting illiquid options, small, < 4 contracts, and infrequent, > 30 minute intervals, orders are more likely to be filled closer to the theoretical price; however, if one's sells are the only trades, traders on the other side will take note and accept ever lowering implied volatilities. With knowledge of what traders will accept, it is always more optimal to trade out of options rather than exercise because of the added costs and uncertainty involved with exercising and liquidating. "Exchange A has 100 shares of a stock at $10, the next 100 shares cost $10.01. Exchange B has the same pricing structure.  A fund manager wants to buy 200 shares of the stock, and decides that buying 100 shares at $10 from each exchange will be cheaper than staying in one exchange and paying $10.01 for the second half of his order.  The manager places two separate orders. Let's say the first order reaches exchange A, and the trade executes at $10. Traders (algorithms) on exchange B see this happen, and adjust their price up to $10.01 accordingly. Now, when the manager's order reaches exchange B, there will no longer be any shares trading at $10.  Some people say that this is front running, but if the manager only wanted 100 shares, the price would have still shifted. Some say this creates a more efficient market with tighter spreads due to the decreased risk to the market maker, but it also means the aggregate bid-ask offers across multiple exchanges are not necessarily accurate, creating a ""false liquidity"". You can decide for yourself whether or not this is a good thing." The situation you're proposing is an over-simplification that wouldn't occur in practice. Orders occur in a sequence over time. Time is an important part of the order matching process. Orders are not processed in parallel; otherwise, the problem of fairness, already heavily regulated, would become even more complex. First, crossed and locked markets are forbidden by regulators.  Crossed orders are where one exchange has a higher bid than another's ask, or a lower ask than another's bid.  A locked market is where a bid on one exchange is equal to the ask on another.  HFTs would be able to make these markets because of the gap between exchange fees. Since these are forbidden, and handling orders in parallel would ensure that a crossed or locked market would occur, orders are serialized (queued up), processed in order of price-time priority. So, the first to cross the market will be filled with the best oldest opposing order. Regulators believe crossed or locked markets are unfair.  They would however eliminate the bid ask spread for many large securities thus the bid-ask cost to the holder. Retail brokers and are generally not members of exchanges and would generally not be members of exchanges unless they are directly routing orders to those exchanges. Most retail brokers charging $7 are considered discount brokers and such brokers route order to Market Makers (who are members of the exchanges). All brokers and market makers must be members of FINRA and must pay FINRA registration and licensing fees.  Discount brokers also have operational costs which include the cost of their facilities, technology, clearing fees, regulation and human capital. Market makers will have the same costs but the cost of technology is probably much higher. Discount brokers will also have market data fees which they will have to pay to the exchanges for the right to show customer real time quotes.  Some of their fees can be offset through payment for order flow (POF) where market makers pay routing brokers a small fee for sending orders to them for execution. The practice of POF has actually allowed retail brokers to keep their costs lower but to to shrinking margins and spread market makers POF has significantly declined over the years.  Markets makers generally do not pass along Exchange access fees which are capped at $.003 (not .0035) to routing brokers.  Also note that The SEC and FINRA charges transactions fees. SEC fee for sales are generally passed along to customers and noted on trade confirms. FINRA TAF is born by the market makers and often subtracted from POF paid to routing firms. Other (full service brokers) charging higher commissions are charging for the added value of their brokers providing advice and expertise in helping investors with investment strategies. They will generally also have the same fees associated with membership of all the exchanges as they are also market makers subject to some of the list of cost mentioned above.  One point of note is that Market Making technology is quite sophisticates and very expensive.  It has driven most of wholesale market makers of the 90s into consolidation.    Retail routing firm's save a significant amount of money for not having to operate such a system (as well as worry about the regulatory headaches associated with running such a system).  This allows them to provide much lower commissions that the (full service) or bulge bracket brokers. How would this trade behave IRL? I don't know how the simulation handles limit orders and bid/ask spreads to know it's feasible, but buying at 4.04 when the current ask is 8.00 seems unlikely.  That would mean that all other sell orders between 8.00 and 4.04 were fulfilled, which means that there were very few sellers or that sell pressure spiked, both of which seem unlikely. In reality, it seems more likely that your order would have sat there until the ask dropped to $4.04 (if it ever did), and then you'd have to wait until the bid rose to $7.89 in order to sell them at that price. However, that kind of swing in option prices in not unrealistic.  Options near at-the-money tend to move in price at about 50% of the change in the underlying, so if amazon suddenly dropped by $5, the option price could drop by $2.60 (from 6.66 to $4.04), and then rise back to $7.89 if the price rose $8 (which would be 1% swing and not unheard of intra-day). But it sounds like you got very lucky (or the simulation doesn't handle option trading realistically) - I've traded options in the past and have had some breaks similar to yours.  I've also had bad breaks where I lost my entire investment (the options expire out-of-the money). So it should be a very limited part of your portfolio, and probably only used for risk management (e.g. buying put options to lock in some gains but keeping some upside potential). "This is a misconception. One of the explanations is that if you buy at the ask price and want   to sell it right away, you can only sell at the bid price. This is incorrect. There are no two separate bid and ask prices. The price you buy (your ""bid"") is the same price someone else sells (their ""sell""). The same goes when you sell - the price you sell at is the price someone else buys. There's no spread with stocks. Emphasized it on purpose, because many people (especially those who gamble on stock exchange without knowing what they're doing) don't understand how the stock market works. On the stock exchange, the transaction price is the match between the bid price and the ask price. Thus, on any given transaction, bid always equals ask. There's no spread. There is spread with commodities (if you buy it directly, especially), contracts, mutual funds and other kinds of brokered transactions that go through a third party. The difference (spread) is that third party's fee for assuming part of the risk in the transaction, and is indeed added to your cost (indirectly, in the way you described). These transactions don't go directly between a seller and a buyer. For example, there's no buyer when you redeem some of your mutual fund - the fund pays you money. So the fund assumes certain risk, which is why there's a spread in the prices to invest and to redeem. Similarly with commodities: when you buy a gold bar - you buy it from a dealer, who needs to keep a stock. Thus, the dealer will not buy from you at the same price: there's a premium on sale and a discount on buy, which is a spread, to compensate the dealer for the risk of keeping a stock."