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question about short selling stocks
My take on this is that with any short-selling contract you are engaging in, at a specified time in the future you will need to transfer ownership of the item(s) you sold to the buyer. Whether you own the item(s) or in your case you will buy your friend's used  car in the meantime (or dig enough gold out of the ground - in the case of hedging a commodity exposure) is a matter of "trust". Hence there is normally some form of margin or credit-line involved to cover for you failing to deliver on expiry.