I think there are plenty of good descriptions of the basics of short selling here, but I wanted to help explain the "who are you borrowing from" aspect. When you trade, you must trade through a broker-dealer. A broker-dealer is authorized to trade on behalf of it's customers. A broker-dealer must uphold certain regulatory requirements put in place by the SEC, and policed by a variety of government and non-government…
Edit: The lender must write off the stock?