What happens when the bid and ask are the same?
This question is impossible answer for all markets but there are 2 more possibilities in my experience: In the world of stock exchanges, the result depends on the market state of the traded stock. There are two possibilities, (a) a trade occurs or (b) no trade occurs. During the so-called auction phase, bid and ask prices may overlap, actually they usually do. During an open market, when bid and ask match, trades occur. In simple terms, this is how the shares are traded, however most of the times market orders are placed. Consider below scenario( hypothetical scenario, there are just 2 traders) Buyer is ready to buy 10 shares @ 5$ and seller is ready to sell 10 shares @ 5.10$, both the orders will remain in open state, unless one wish to change his price, this is an example of limit order. Market orders If seller is ready to sell 10 shares @ 5$ and another 10 shares @5.05$, if buyer wants to buy 20 shares @ market price, then the trade will be executed for 10 shares @ 5$ and another 10 shares @ 5.05$ Rule 610 (Google for it) stands that if Bid and Ask are the same, the market is considered Locked, and the exchange must stop all trading. So the same person can't quote the same bid and ask price. However, HFTs have found ways to circumvent this limitation when exchanges created special order types for them, e.g. Spam-and-Cancel
It is unlikely that buying 100 shares will have any effect on a stock's price, unless the stock's average trading volume is incredibly low.  That being said, no matter how many share you buy, there's no way to know what the impact on the price will be, because that's only one factor in how shares are priced.  If anyone could figure out the answer to your question then they'd be extremely rich, because they'd simply watch for big share trades and then buy those stocks on the way up. The market makers who actually execute the trades are the ones who set the prices, and most stocks have multiple market makers trading the stock, so the bid/ask you see is the highest bid and lowest ask.  The market makers set the price based on what the trend of the stock is.  If, for instance, there's a large number of sell orders against a stock, the market makers will start dropping the bid prices as they fill execution orders, and as they see buy orders increase, they'll raise ask prices as they fill execution orders.  The market makers earn the difference between what they paid to buy someone's stock who was selling and what they get from someone else who buys it. This is a simplified explanation, so pro traders, don't beat me up! (grin) So, basically, it takes quite a bit of share volume in one direction or another to affect a stock's price.  I can guarantee a 100-share trade wouldn't even be noticed by market makers. I hope this helps. Good luck! "In a sense, yes. There's a view in Yahoo Finance that looks like this   For this particular stock, a market order for 3000 shares (not even $4000, this is a reasonably small figure) will move the stock past $1.34, more than a 3% move.  Say, on the Ask side there are 100,000 shares, all with $10 ask. It would take a lot of orders to purchase all these shares, so for a while, the price may stay right at $10, or a bit lower if there are those willing to sell lower.  But, say that side showed $10 1000, $10.25 500, $10.50 1000. Now, the volume is so low that if I decided I wanted shares at any price, my order, a market order will actually drive the market price right up to $10.50 if I buy 2500 shares ""market"".  You see, however, even though I'm a small trader, I drove the price up. But now that the price is $10.50 when I go to sell all 2500 at $10.50, there are no bids to pay that much, so the price the next trade will occur at isn't known yet. There may be bids at $10, with asking (me) at $10.50. No trades will happen until a seller takes the $10 bid or other buyers and sellers come in." "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." The Bid price is simply the highest buy price currently being offered and the Ask price simply the lowest sell price being offered. The list of Bid and Ask prices is called the market depth. When the Bid and Ask prices match then a sale goes through. When looking to sell you would generally look at both the Bid and Ask prices. As a seller you want to be matched with the Bid price to get a sale, but you also need to check the current list of Ask prices. If the price you want to sell at is too high you will be placed down the Ask price list, and unless the price moves up to match your sell price you will not end up selling. On the other-hand, if your price to sell is too low and in fact much lower than the current lowest sell price you may get a quick sale but maybe at a lower price than you could have gotten. Similarly, when looking to buy, you would generally also look at both the Bid and Ask prices. As a buyer you want to be matched with the Ask price to get a sale, but you also need to check the current list of Bid prices. If the price you want to buy at is too low you will be placed down the Bid price list, and unless the price moves down to match your buy price you will no end up buying. On the other-hand, if your price to buy is too high and in fact much higher than the current highest buy price you may get a quick purchase but maybe at a higher price than you could have gotten. So, whether buying or selling, it is important to look at and consider both the Bid and Ask prices in the market depth. Limit books are managed by exchanges. If an order is not immediately filled, it is sent to the book.  From there, orders are generally executed on price-time-priority.  The one major exception is the precedence hide-not-slide orders have over earlier placed visible slidden limit orders since unslidden orders are treated like a modification/cancellation. To an exchange, a modification is the same as a cancellation since it charges no fees for placing or canceling orders, only for trades.  The timestamp is reset, and price-time-priority is applied in the same way, so if a modified order isn't immediately filled, it is sent back to the book to be filled in order of price-time-priority. "Both prices are quotes on a single share of stock. The bid price is what buyers are willing to pay for it. The ask price is what sellers are willing to take for it. If you are selling a stock, you are going to get the bid price, if you are buying a stock you are going to get the ask price. The difference (or ""spread"") goes to the broker/specialist that handles the transaction." 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. I'm not an economics expert or anything, but what seems strange to me is the randomising part. Everything else looks fine but why does it randomly select the next item for price increase, wouldn't this depend on supply/demand and the players in the game?