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Why does selling and then rebuying stock not lead to free money?
You are misunderstanding what makes the price of a stock go up and down. Every time you sell a share of a stock, there is someone else that buys the stock.  So it is not accurate to say that stock prices go down when large amounts of the stock are sold, and up when large amounts of the stock are bought.  Every day, the amount of shares of a stock that are bought and sold are equal to each other, because in order to sell a share of stock, someone has to buy it. Let me try to explain what actually happens to the price of a stock when you want to sell it. Let's say that a particular stock is listed on the ticker at $100 a share currently.  All this means is that the last transaction that took place was for $100; someone sold their share to a buyer for $100. Now let's say that you have a share of the stock you'd like to sell.  You are hoping to get $100 for your share.  There are 2 other people that also have a share that they want to sell.  However, there is only 1 person that wants to buy a share of stock, and he only wants to pay $99 for a share.  If none of you wants to sell lower than $100, then no shares get sold.  But if one of you agrees to sell at $99, then the sale takes place.  The ticker value of the stock is now $99 instead of $100. Now let's say that there are 3 new people that have decided they want to buy a share of the stock.  They'd like to buy at $99, but you and the other person left with a share want to sell at $100.  Either one of the sellers will come down to $99 or one of the buyers will go up to $100.  This process will continue until everyone that wants to sell a share has sold, and everyone who wants to buy a share has bought.  In general, though, when there are more people that want to sell than buy, the price goes down, and when there are more people that want to buy than sell, the price goes up. To answer your question, if your selling of the stock had caused the price to go down, it means that you would have gotten less money for your stock than if it had not gone down.  Likewise, if your buying the stock had caused it to go up, it just means that it would have cost you more to buy the stock.  It is just as likely that you would lose money doing this, rather than gain money.