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system
question about short selling stocks
user
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.
assistant