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Why does short selling require borrowing?
This can be best explained with an example. Bob thinks the price of a stock that Alice has is going to go down by the end of the week, so he borrows a share at $25 from Alice. The current price of the shares are $25 per share. Bob immediately sells the shares to Charlie for $25, it is fair, it is the current market price. A week goes by, and the price does fall to $20. Bob buys a share from David at $20. This is fair, it is the current market value. Then Bob gives the share back to Alice to settle what he borrowed from her, one share. Now, in reality, there is interest charged be Alice on the borrowed value, but to keep it simple, we'll say she was a friend and it was a zero interest loan. So then Bob was able to sell something he didn't own for $25 and return it spending $20 to buy it, settling his loan and making $5 in the transaction. It is the selling to Charlie and buying from David (or even Charlie later, if he decided to dump the shares), without having invested any of your own money that earns the profit.