Howto choose a marketplace while submitting an order for a stock trade
It depends on your cost structure and knowledge of the exchanges. It could be optimal to make a manual exchange selection so long as it's cheaper to do so. For brokers with trade fees, this is a lost cause because the cost of the trade is already so high that auto routing will be no cheaper than manual routing. For brokers who charge extra to manually route, this could be a good policy if the exchange chosen has very high rebates.  This does not apply to equities because they are so cheap, but there are still a few expensive option exchanges. This all presumes that one's broker shares exchange rebates which nearly all do not. If one has direct access to the exchanges, they are presumably doing this already. To do this effectively, one needs: For anyone trading with brokers without shared rebates or who does not have knowledge of the exchange prices and their liquidities, it's best to auto route.
A company typically goes public in order to bring in additional capital. In an IPO, the company (through its officials) will typically do so by issuing additional shares, and offering to sell those to investors. If they did not do that, then there would be no net capital gain for the company; if person A sells share in company C to person B, then company C does not benefit directly from the exchange. By issuing and selling additional shares, the total value of all stock in the company can increase. Being publicly traded also greatly increases the confidence in the valuation of the company, as a consequence of the perfect market theory. There is nothing in this that says that initial investors (cofounders, employees, etc.) need to sell their shares in the process. They might choose to do so, or they might not; or they might be prevented from doing so by terms of any agreements that they have signed or by insider trading laws. Compare What happens to internal stock when a company goes public? Depending on specifics, it might be reasonable for the company to perform a share split prior to the initial public offering. That, however, doesn't affect the total value of the shares, only the price per share. "Firstly, if a stock costs $50 this second, the bid/ask would have to be 49/50.  If the bid/ask were 49/51, the stock would cost $51 this second.  What you're likely referring to is the last trade, not the cost.  The last trading price is history and doesn't apply to future transactions. To make it simple, let's define a simple order book.  Say there is a bid to buy 100 at $49, 200 at $48, 500 at $47.  If you place a market order to sell 100 shares, it should all get filled at $49.  If you had placed a market order to sell 200 shares instead, half should get filled at $49 and half at $48.  This is, of course, assuming no one else places an order before you get yours submitted.  If someone beats you to the 100 share lot, then your order could get filled at lower than what you thought you'd get.  If your internet connection is slow or there is a lot of latency in the data from the exchange, then things like this could happen.  Also, there are many ECNs in addition to the exchanges which may have different order books.  There are also trades which, for some reason, get delayed and show up later in the ""time and sales"" window. But to answer the question of why someone would want to sell low... the only reason I could think is they desire to drive the price down." "There is no unique identifier that exists to identify specific shares of a stock. Just like money in the bank, there is no real reason to identify which exact dollar bills belong to me or you, so long as there is a record that I own X bills and I can access them when I want. (Of course, unlike banks, there is still a 1:1 relationship between the amount I should own and the amount they actually hold). If I may reach a bit, the question that I assume you are asking is how are shared actually tracked, transferred, and recorded so that I know for certain that I traded you 20 Microsoft shares yesterday and they are now officially yours, given that it's all digital. While you can technically try and request a physical share certificate, it's very cumbersome to handle and transfer in that form.  Ownership of shares themselves are tracked for brokerage firms (in the case of retail trading, which I assume is the context of this question as we're discussion personal finance). Your broker has a record of how many shares of X, Y, and Z you own, when you bought each share and for how much, and while you are the beneficial owner of record (you get dividends, voting rights, etc.) your brokerage is the one who is ""holding"" the shares. When you buy or sell a stock and you are matched with a counterparty (the process of which is beyond the scope of this question) then a process of settlement comes into play. In the US, settlement takes 3 working days to process, and technically ownership does not transfer until the 3rd day after the trade is made, though things like margin accounts will allow you to effectively act as if you own the shares immediately after a buy/sell order is filled. Settlement in the US is done by a sole source, the Depository Trust & Clearing Corporation (DTCC). This is where retail and institutional trade all go to be sorted, checked and confirmed, and ultimately returned to the safekeeping of their new owners' representatives (your brokerage). Interestingly, the DTCC is also the central custodian for shares both physical and virtual, and that is where the shares of stock ultimately reside." In addition to the higher risk as pointed out by @JamesRoth, you also need to consider that there are regulations against 'naked shorting' so you generally need to either own the security, or have someone that is willing to 'loan' the security to you in order to sell short.  If you own a stock you are shorting, the IRS could view the transaction as a Sell followed by a buy taking place in a less than 30 day period and you could be subject to wash-sale rules.  This added complexity (most often the finding of someone to loan you the security you are shorting) is another reason such trades are considered more advanced. You should also be aware that there are currently a number of proposals to re-instate the 'uptick rule' or some circuit-breaker variant. Designed to prevent short-sellers from driving down the price of a stock (and conducting 'bear raids etc) the first requires that a stock trade at the same or higher price as prior trades before you can submit a short.  In the latter shorting would be prohibited after a stock price had fallen a given percentage in a given amount of time.  In either case, should such a rule be (re)established then you could face limitations attempting to execute a short which you would not need to worry about doing simple buys or sells.   As to vehicles that would do this kind of thing (if you are convinced we are in a bear market and willing to take the risk) there are a number of ETF's classified as 'Inverse Exchange Traded Funds (ETF's) for a variety of markets that via various means seek to deliver a return similar to that of 'shorting the market' in question.  One such example for a common broad market is ticker SH the ProShares Short S&P500 ETF, which seeks to deliver a return that is the inverse of the S&P500 (and as would be predicted based on the roughly +15% performance of the S&P500 over the last 12 months, SH is down roughly -15% over the same period).  The Wikipedia article on inverse ETF's lists a number of other such funds covering various markets.   I think it should be noted that using such a vehicle is a pretty 'aggressive bet' to take in reaction to the belief that a bear market is imminent.  A more conservative approach would be to simply take money out of the market and place it in something like CD's or Treasury instruments.  In that case, you preserve your capital, regardless of what happens in the market. Using an inverse ETF OTOH means that if the market went bull instead of bear, you would lose money instead of merely holding your position. "Well, they don't have to enter a market order. I was just saying that for your benefit.   In practice what will really happen is they'll say to themselves   ""I want ABC for no more than 70.00""  The current bid is at 69.60 They'll use a limit order on 10,000 shares. If anyone offers them more than 70.00 the computer program they're using will not accept.  However, the shares won't be bought all at once. A limit order for 10,000 shares will be spit into 1,000 shares at 69.70 and 10,000 for 69.65  The HFT can then make a reasonable guess someone is buying in bulk. So then the HFT sends out a sell order saying ""10 shares at 70.10"". The guy doesn't want that so they do it over again at 70.09  Eventually they notice when the guy accepts at 70.00  So now they knew this guy is willing to pay that. Then HFT buys all the shares they can. ALL SHARES, not just the amount the guy wants. They buy it all at 69.95, which is higher than the 69.70 the guy just paid or whatever.  They then sell it all back to the guy at 70.00  The guy has no choice because all the shares are now gone. He has to pay his highest price.   Once the guy gets his order the HFT just dumps the rest of the shares back at the price they bought them at.  edit: Another way to pull this off is to just literally build your own exchange. Then you can do all of this without any clever tricks. You can just outright give yourself preference over everyone else because you can truly decide what order the trades are being executed." There is no one answer to this question, but there are some generalities. Most exchanges make a distinction between the passive and the aggressive sides of a trade.  The passive participant is the order that was resting on the market at the time of the trade.  It is an order that based on its price was not executable at the time, and therefore goes into the order book.  For example, I'm willing to sell 100 shares of a stock at $9.98 but nobody wants to buy that right now, so it remains as an open order on the exchange. Then somebody comes along and is willing to meet my price (I am glossing over lots of details here).  So they aggressively take out my order by either posting a market-buy, or specifically that they want to buy 100 shares at either $9.98, or at some higher price. Most exchanges will actually give me, as the passive (i.e. liquidity making) investor a small rebate, while the other person is charged a few fractions of a cent.  Google found NYSEArca details, and most other exchanges make their fees public as well.  As of this writing the generic price charged/credited: But they provide volume discounts, and many of the larger deals do fall into another tier of volume, which provides a different price structure. You typically need to specify that you want the GTC order to be working during the Extended hours session. I trade on TD Ameritrade's Thinkorswim platform, and you can select DAY, GTC, EXT or GTC_EXT. So in your case, you would select GTC_EXT. It depends on the way you have directed the order and the execution agreement you have signed with your broker. In case of DMA (direct market access) you would direct your order to the specific exchange - and that exchange would post your offer, assuming you did not tag it as hidden. However, if you just gave your order to the broker (be it via telephone, email or even online), they may not have to display your order to the market or chose which exchange to sell it on. It will also depend where the stock is listed. For most US listed and OTC stocks, regulation NMS applies where your order should have been executed against if it went to the exchanges. Check your account opening docs and agreements, particulary the execution agreement. In there it will tell you how your order should be treated. In case where the broker stipulates that you have DMA or that they will direct your order to Lit markets (public exchanges and not market making firms and dark-pools) then you may have a case - you would need to request information to whcih exchange your broker sent the order to. In case that you gave them discretion on routing of your order - read the fine print. The answer lies there. Regarding NBBO missing you quote as quantycuenta suggested above is also a possibility, however Reg NMS should take care of this. Do you have stock and date & time of your order?