Why are there hidden bids and offers in the US stock market for the more illiquid stocks?
When you place a bid between the bid/ask spread, that means you are raising the bid (or lowering the ask, if you are selling). The NBBO (national best bid and offer) is now changed because of your action, and yes, certain kinds of orders may be set to react to that (a higher bid or lower ask triggering them), also many algorithms (that haven't already queued an order simply waiting for a trigger, like in a stop limit) read the bid and ask and are programmed to then place an order at that point. "Certain brokers allow for hidden orders to be placed in the market. It is as simple as that. Refer to Interactive Brokers as one example. If you press on the "" i "" next to ""Hidden"" you will get the following description.  Some brokers may represent the hidden orders by an * next to the price level. Sometimes large orders are place as these hidden orders to avoid large movements in the stock price (especially if the stock is illiquid as per your observation)."
"When you place a limit sell order of $10.00 (for a stock on an option) you are adding your order to the book.  Anyone who places a buy at-the-market or with a limit price over $10.00 will have that order immediately fulfilled through the offer you have placed on the book. On the other hand, if that other person places a buy for $8.00, then the spread will now be ""$8.00 bid, $10.00 ask"". Priority is based on first the price (all $9.99 asks will clear before $10.00) and within each bucket this is based on the time your order was submitted. This is why in bidding markets (including eBay) buying at $x.01 is way better than $x.00 and selling at $x.99 is better than $(x+1).00. Source: https://en.wikipedia.org/wiki/Order_(exchange) under ""first-come-first-served""" When a stock is ask for 15.2 and bid for 14.5, and the last market price was 14.5, what does it mean? It means that the seller wants to sell for a higher price than the last sale while the buyer does not want to buy for more than the last sale price. Or what if the last price is 15.2? The seller is offering to sell for the last sale price, but the buyer wants to buy for less. There is economic value added to the marketplace, by having many investors trading stocks. The stock market itself can be thought of as a tool which provides additional 'liquidity' to the marketplace. Liquidity is the ease with which you can convert your assets into cash (for example, how quickly could you sell your car if you needed money to pay a medical bill?). Without a stock market, funds would be very illiquid - an investor would likely need to post advertisements to have other people consider buying his/her shares. Until the match between a buyer and seller is found, the person with the shares can't use the cash they need. On the other side of the transaction, are people who have an appetite for risk. This means that, for various reasons, they are willing to take on more risk than you, if it pays off on average (they are young [and have many years of salary earnings in front of them], or they are rich [can afford to lose money sometimes if it pays off on average]). Consider this like a transaction between your insurance broker - you don't want to pay for a new car if you get in an accident, and you're willing to pay total annual premiums that, on average, will cost more than that same car over time. You don't want the risk, but the insurance company does - that's how they make money. So by participating in any marketplace, you are providing value, in the form of liquidity, and by allowing the market to allocate risk to those willing to take it on. "If we can agree that 2010 was closer to the low of 2009 than 2007 then the rich did all the buying while the super-rich did all the selling. http://www2.ucsc.edu/whorulesamerica/power/wealth.html Looks like the rich cleaned up during the Tech Crash too, but it looks like the poor lost faith. That limited data makes it look like the best investors are the rich. Market makers are only required by the exchanges to provide liquidity, bids & asks.  They aren't required to buy endlessly. In fact, market makers (at least the ones who survive the busts) try to never have a stake in direction.  They do this by holding equal inventories of long and shorts.  They are actually the only people legally allowed to naked short stock: sell without securing shares to borrow.  All us peons must secure borrowed shares before selling short. Also, firms involved in the actual workings of the market like bookies but unlike us peons who make the bets play by different margin rules.  They're allowed to lever through the roof because they take on low risk or near riskless trades and ""positions"" (your broker, clearing agent, etc actually directly ""own"" your financial assets and borrow & lend them like a bank). http://www.finra.org/web/groups/industry/@ip/@reg/@notice/documents/notices/p004001.pdf This is why market makers can be assumed not to load up on shares during a decline; they simply drop the bids & asks as their bids are hit." "I'm just trying to visualize the costs of trading. Say I set up an account to trade something (forex, stock, even bitcoin) and I was going to let a random generator determine when I should buy or sell it. If I do this, I would assume I have an equal probability to make a profit or a loss. Your question is what a mathematician would call an ""ill-posed problem.""  It makes it a challenge to answer.  The short answer is ""no.""  We will have to consider three broad cases for types of assets and two time intervals. Let us start with a very short time interval.  The bid-ask spread covers the anticipated cost to the market maker of holding an asset bought in the market equal to the opportunity costs over the half-life of the holding period.  A consequence of this is that you are nearly guaranteed to lose money if your time interval between trades is less than the half-life of the actual portfolio of the market maker.  To use a dice analogy, imagine having to pay a fee per roll before you can gamble.  You can win, but it will be biased toward losing. Now let us go to the extreme opposite time period, which is that you will buy now and sell one minute before you die.  For stocks, you would have received the dividends plus any stocks you sold from mergers.  Conversely, you would have had to pay the dividends on your short sales and received a gain on every short stock that went bankrupt.  Because you have to pay interest on short sales and dividends passed, you will lose money on a net basis to the market maker.  Maybe you are seeing a pattern here.  The phrase ""market maker"" will come up a lot. Now let us look at currencies.  In the long run, if the current fiat money policy regime holds, you will lose a lot of money.  Deflation is not a big deal under a commodity money regime, but it is a problem under fiat money, so central banks avoid it.  So your long currency holdings will depreciate.  Your short would appreciate, except you have to pay interest on them at a rate greater than the rate of inflation to the market maker. Finally, for commodities, no one will allow perpetual holding of short positions in commodities because people want them delivered.  Because insider knowledge is presumed under the commodities trading laws, a random investor would be at a giant disadvantage similar to what a chess player who played randomly would face against a grand master chess player.  There is a very strong information asymmetry in commodity contracts.  There are people who actually do know how much cotton there is in the world, how much is planted in the ground, and what the demand will be and that knowledge is not shared with the world at large.  You would be fleeced. Can I also assume that probabilistically speaking, a trader cannot do   worst than random? Say, if I had to guess the roll of a dice, my   chance of being correct can't be less than 16.667%. A physicist, a con man, a magician and a statistician would tell you that dice rolls and coin tosses are not random.  While we teach ""fair"" coins and ""fair"" dice in introductory college classes to simplify many complex ideas, they also do not exist.  If you want to see a funny version of the dice roll game, watch the 1962 Japanese movie Zatoichi.  It is an action movie, but it begins with a dice game.   Consider adopting a Bayesian perspective on probability as it would be a healthier perspective based on how you are thinking about this problem.  A ""frequency"" approach always assumes the null model is true, which is what you are doing.  Had you tried this will real money, your model would have been falsified, but you still wouldn't know the true model. Yes, you can do much worse than 1/6th of the time.  Even if you are trying to be ""fair,"" you have not accounted for the variance. Extending that logic, then for an inexperienced trader, is it right to   say then that it's equally difficult to purposely make a loss then it   is to purposely make a profit? Because if I can purposely make a loss,   I would purposely just do the opposite of what I'm doing to make a   profit. So in the dice example, if I can somehow lower my chances of   winning below 16.6667%, it means I would simply need to bet on the   other 5 numbers to give myself a better than 83% chance of winning. If the game were ""fair,"" but for things like forex the rules of the game are purposefully changed by the market maker to maximize long-run profitability.  Under US law, forex is not regulated by anything other than common law.  As a result, the market maker can state any price, including prices far from the market, with the intent to make a system used by actors losing systems, such as to trigger margin calls.  The prices quoted by forex dealers in the US move loosely with the global rates, but vary enough that only the dealer should make money systematically.  A fixed strategy would promote loss. You are assuming that only you know the odds and they would let you profit from your 83.33 percentage chance of winning. So then, is the costs of trading from a purely probabilistic point of   view simply the transaction costs? No matter what, my chances cannot   be worse than random and if my trading system has an edge that is   greater than the percentage of the transaction that is transaction   cost, then I am probabilistically likely to make a profit? No, the cost of trading is the opportunity cost of the money.  The transaction costs are explicit costs, but you have ignored the implicit costs of foregone interest and foregone happiness using the money for other things. You will want to be careful here in understanding probability because the distribution of returns for all of these assets lack a first moment and so there cannot be a ""mean return.""  A modal return would be an intellectually more consistent perspective, implying you should use an ""all-or-nothing"" cost function to evaluate your methodology." Differences in liquidity explain why American-style options are generally worth more than their European-style counterparts. As far as I can tell, no one mentioned liquidity in their answer to this question, they just introduced needlessly complex math and logic while ignoring basic economic principles. That's not to say the previous answers are all wrong - they just deal with periphery factors instead of the central cause.  Liquidity is a key determinant of pricing/valuation in financial markets. Liquidity simply describes the ease with which an asset can be bought and sold (converted to cash). Without going into the reasons why, treasury bills are one of the most liquid securities - they can be bought or sold almost instantly at any time for an exact price. The near-perfect liquidity of treasuries is one of the major reasons why the price (yield) of a t-bill will always be higher (lower yield) than that of an otherwise identical corporate or municipal bond. Stated in general terms, a relatively liquid asset is always worth more than an relatively illiquid asset, all else being equal.  The value of liquidity is easy to understand - we experience it everyday in real life. If you're buying a house or car, the ability to resell it if needed is an important component of the decision. It's the same for investors - most people would prefer an asset that they can quickly and easily liquidate if the need for cash arises.  It's no different with options. American-style options allow the holder to exercise (liquidate) at any time, whereas the buyer of a European option has his cash tied up until a specific date. Obviously, it rarely makes sense to exercise an option early in terms of net returns, but sometimes an investor has a desperate need for cash and this need outweighs the reduction in net profits from early exercise.  It could be argued that this liquidity advantage is eliminated by the fact that you can trade (sell) either type of option without restriction before expiration, thus closing the long position. This is a valid point, but it ignores the fact that there's always a buyer on the other side of an option trade, meaning the long position, and the right/restriction of early exercise, is never eliminated, it simply changes hands. It follows that the American-style liquidity advantage increases an options market value regardless of one's position (call/put or short/long). Without putting an exact number on it, the general interest rate (time value of money) could be used to approximate the additional cost of an American-style option over a similar European-style contract. I'd think that liquidity and speed are prioritized (even over retail brokers and in come cases over PoP) for institutional traders who by default have large positions.  When the going gets tough, these guys are out and the small guys - trading through average retail brokers - are the ones left holding the empty bag. Everyone has a price. If nobody is selling shares, then increase the price you will buy them for. And then wait. Somebody will have some hospital bills to pay for eventually. I buy illiquid investments all the time, and thats typically what happens. Great companies do not have liquidity problems.