AFAIK short selling originated as a flaw in manually settled stock exchanges. ( You only needed to have the stocks when the accounts were settled, so you could sell first and then buy later.) But you can get the same effect by borrowing bitcoins and selling them. Then you need to buy them back later.
That's exactly what short selling stock is: you borrow it from someone and sell it, and return it to them later by buying it for (hopefully) cheaper. To sell short without borrowing is called 'naked' short selling, and it's not allowed on US equity exchanges.
