Interestingly, I read that it is extremely difficult to do short selling with stock you own, because it is a way to avoid taxes. If you own a stock for 5 months, and think it is ready to tank, but you don’t want to pay short-term capital gains, you borrow someone else’s stock, and short-sell it. IN a couple of months, you can deliver your shares to them, and you’ve got long-term gains instead of short-term.
See: “Covered puts and calls”.