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question about short selling stocks
If you had an agreement with your friend such that you could bring back a substantially similar car, you could sell the car and return a different one to him.  The nature of shares of stock is that, within the specified class, they are the same.  It's a fungible commodity like one pound of sand or a dollar bill.  The owner doesn't care which share is returned as long as a share is returned.   I'm sure there's a paragraph in your brokerage account terms of service eluding to the possibility of your shares being included in short sale transactions.