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Short selling on its own is trading on the hope that a stock goes down by borrowing shares from someone who owns them and immediately selling them. Your broker will match you to an owner who is willing to lend the shares if there are any available. To exit the trade, you buy back the shares you borrowed.

Your profit from this trade is the stock's price when you entered the trade minus the price when you buy it back (i.e. you want to sell high and then buy low), less any financing costs from borrowing the shares.