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Why does selling and then rebuying stock not lead to free money?
There are a few people that do this for a living. They are called "market makers" or "specialists" in a particular stock. First of all, this requires a lot of capital. You can get burned on a few trades, a process known as "gambler's ruin," but if you have enough capital to weather the storm, you can make money. Second, you have to be "licensed" by the stock market authorities, because you need to have stock market trading experience and other credentials. Third, you are not allowed to buy and sell at will. In order to do your job, you have to "balance the boat," that is buy, when others are selling, and sell, when others are buying, in order to keep the market moving in two directions. It's a tough job that requires a lot of experience, plus a license, but a few people can make a living doing this.