Buy the open and set a 1% limit sell order
Nothing is wrong and it should be profitable - but it sounds too good to be true. The devil is in the details and you have not described how you found those stocks. For example, you may have scanned the 500 stocks in the S&P 500, and you may have found a few that exhibit that pattern over a given time window. But it doesn't mean that they will continue to do so. In other words they may just be random outliers. This is generically called overfitting. A more robust test would be to use a period, say 2000-2005 to find those stocks and check over a future period, say 2006-2014 if the strategy you describe is profitable. My guess is that it won't. One should also point out that you make a major assumption in that the high of the day doesn't occur on a gap up in morning trading. It's unlikely that you'd fill at a reasonable price, thereby throwing your strategy into disarray.
Actually the insertion part is interesting because they actually aren't middlemen just faster buyer's and sellers on the open market. They just buy what one person is selling and sell it to someone else who is at the same time looking to buy. So I guess the issue is the 'same time' isn't the same for all parties concerned. On the other hand both buyer and seller can set limits on the bounds of the price and only transact when someone meets their terms. If you are willing to accept 'whatever the market will bear' then it seems like you should be OK with getting it. The opening price is derived from new information received. It reflects the current state of the market. Opening Price Deviation (from Investopedia): Investor expectation can be changed by corporate announcements or   other events that make the news. Corporations typically make   news-worthy announcements that may have an effect on the stock price   after the market closes. Large-scale natural disasters or man-made   disasters such as wars or terrorist attacks that take place in the   afterhours may have similar effects on stock prices. When this   happens, some investors may attempt to either buy or sell securities   during the afterhours. Not all orders are executed during after-hours trading. The lack of   liquidity and the resulting wide spreads make market orders   unattractive to traders in after-hours trading. This results in a   large amount of limit or stop orders being placed at a price that is   different from the prior day’s closing price. Consequently, when the   market opens the next day, a substantial disparity in supply and   demand causes the open to veer away from the prior day’s close in the   direction that corresponds to the effect of the announcement, news or   event. "I've alway thought that it was strange, but the ""price"" that gets quoted on a stock exchange is just the price of the last transaction. The irony of this definition of price is that there may not actually be any more shares available on the market at that price. It's also strange to me that the price isn't adjusted at all for the size of the transaction. A transaction of just 1 share will post a new price even if just seconds earlier 100,000 shares traded for a different price. (Ok, unrealistic example, but you get my point.) I've always believed this is an odd way to describe the price. Anyway, my diatribe here is supposed to illustrate the point that the fluctuations you see in price don't really reflect changing valuations by the stock-owning public. Each post in the exchange maintains a book of orders, with unmatched buy orders on one side and unmatched sell orders on the other side. If you go to your broker and tell him, ""fill my order for 50,000 shares at market price"", then the broker won't fill you 50,000 shares at .20. Instead, he'll buy the 50 @ .22, then 80 @ .23, then 100 @ .30, etc. Because your order is so large compared to the unmatched orders, your market order will get matched a bunch of the unmatched orders on the sell side, and each match will notch the posted price up a bit. If instead you asked the broker, ""open a limit order to buy 50000 shares at .20"", then the exchange will add your order to the book: In this case, your order likely won't get filled at all, since nobody at the moment wants to sell at .20 and historically speaking it's unlikely that such a seller will suddenly appear. Filling large orders is actually a common problem for institutional investors: http://www.businessweek.com/magazine/content/05_16/b3929113_mz020.htm http://www.cis.upenn.edu/~mkearns/papers/vwap.pdf (Written by a professor I had in school!)" "Market price is just the bid or offer price of the last sell or buy order in the market. The price that you actually receive or pay will be the price that the person buying the stock off you or selling it to you will accept. If there are no other participants in the market to make up the other side of your order (i.e. to buy off you if you are selling or to sell to you if you are buying) the exchange pays large banks to be ""market makers""; they fulfil your order using stocks that they don't want to either buy or sell just so that you get your order filled. When you place an order outside of market hours the order is kept on the broker's order books until the market reopens and then, at market opening time there is an opening ""auction"" at which orders are matched to opposing orders (i.e. each buy order will be matched with a sell) at a price determined by auction. You will not know what price the order was filled at until it has been filled. If you want to guarantee a price you can do so by placing a limit order that says not to pay more than a certain price for any unit of the stock." A CFD broker will let you open a trade on margin as long as your account balance is more than the margin required on all your open trades. If the required margin increases within a certain percentage of your account balance, you will get a margin call. If you then don't deposit more funds or close losing trades out, the broker will close all your trades. Note: Your account balance is the remaining funds you have left to open new trades with. I always use stop loss orders with all my open trades, and because of this my broker reduces the amount of margin required on each trade. This allows me to have more open trades at the one time without increasing my funds. Effects of a Losing Trade on Margin Say I have an account balance of $2,000 and open a long trade in a share CFD of 1,000 CFDs with a share price of $10 and margin of 10%. The face value of the shares would be $10,000, but my initial margin would be $1,000 (10% of $10,000).  If I don't place a stop loss and the price falls to $9, I would have lost $1,000 and my remaining margin would now be $900 (10% of $9,000). So I would have $100 balance remaining in my account. I would probably receive a margin call to deposit more funds in or close out my trade. If I don't respond the broker will close out my position before my balance gets to $0. If instead I placed a stop loss at say $9.50, my initial margin might be reduced to $500. As the price drops to $9.60 I would have lost $400 and my remaining margin would now only be $100, with my account balance at $1,500. When the price drops to $9.50 I will get stopped out, my trade will be closed and I would have lost $500, with my account balance still at $1,500. Effects of a Winning Trade on Margin Say I have the same account balance as before and open the same trade but this time the price moves up. If I don't place a stop loss and the price goes up to $11, I would have made a $1,000 profit and my remaining margin will now be $1,100 (10% of $11,000). So my account balance would now be $2,000 + $1,000 - $1,100 = $1,900. If I had placed a stop loss at say $9.50 again and the price moves up to $10.50, I would have made a profit of $500 and my margin would now be $1,000. My account balance would be $2,000 + $500 - $1,000 = $1,500. However, if after the price went up to $10.50 I also moved my stop loss up to $10, then I would have $500 profit and only $500 margin. So my balance in this case would be $2,000 + $500 - $500 = $2,000. So by using stop losses as part of your risk management you can reduce the margin used from your balance which will allow you to open more trades without any extra funds deposited into your account. "You can place the orders like you suggested.  This would be useful in a market that is moving quickly where you want to be reasonably sure of execution but don't want the full exposure of a market order.  This won't jump your spot in the queue though in the sense that you won't get ahead of other orders that are ""ready"" for execution just because you have crossed the spread aggressively." "A market sell order will be filled at the highest current ""bid"" price. For a reasonably liquid stock, there will be several buy orders in line, and the highest bid must be filled first, so there should a very short time between when you place the order and when it is filled.   What could happen is what's called front running. That's when the broker places their own order in front of yours to fulfill the current bid, selling their own stock at the slightly higher price, causing your sale to be filled at a lower price.  This is not only unethical but illegal as well. It is not something you should be concerned about with a large broker.   You should only place a market order when you don't care about minute differences between the current ask and your execution price, but want to guarantee order execution. If you absolutely have to sell at a minimum price, then a limit order is more appropriate, but you run the risk that your limit will not be reached and your order will not be filled. So the risk is a tradeoff between a guaranteed price and a guaranteed execution." "Your logic breaks down because you assume that you are the only market participant on your side of the book and that the participant on the other side of the book has entered a market order. Here's what mostly happens: Large banks and brokerages trading with their own money (we call it proprietary or ""prop"" trading) will have a number of limit (and other, more exotic) orders sitting on both sides of the trading book waiting to buy or sell at a price that they feel is advantageous. Some of these orders will have sat on the book for many months if not years. These alone are likely to prevent your limit orders executing as they are older so will be hit first even if they aren't at a better price.  On more liquid stocks there will also be a number of participants entering market orders on both sides of the book whose orders are matched up before limit orders are matched with any market orders. This means that pairing of market orders, at a better price, will prevent your limit order executing. In many markets high frequency traders looking for arbitrage opportunities (for example) will enter a few thousand orders a minute, some of these will be limit orders just off touch, others will be market orders to be immediately executed. The likelihood that your limit order, being as it is posited way off touch, is hit with all those traders about is minimal. On less liquid stocks there are market makers (large institutional traders) who effectively set the bid and offer prices by being willing to provide liquidity and fill the market orders at a temporary loss to themselves and will, in most cases, have limit orders set to provide this liquidity that will be close to touch. They are paid to do this by the exchange and inter-dealer brokers through their fees structure. They will fill the market orders that would hit your limit if they think that it would provide more liquidity in such a way that it fulfils their obligations. Only if there are no other participants looking to trade on the instrument at a better price than your limit (which, of course they can see unless you enter it into a dark pool) AND there is a market order on the opposite side of the book will your limit order be instantaneously be hit, executed, and move the market price."