Quote from loufah:
I can't see how selling stock that you didn't buy could do anything but complicate matters. One of my brokers (the same one I mentioned in a previous post) once accidentally placed a $40K buy order and fill into my account. The transactions appeared in my execution log as I watched, and I called them within minutes, but this was already after hours and nobody looked at it so I had to badger them again the next morning. I was pretty sure it was a mistake and not fraud because the timestamps were several hours old, so it seemed that someone doing end-of-day cleanup, maybe moving something from a margin account to a cash account, mistyped an account number. And that is indeed what happened. Still, it blew out my margin and I decided to sell one of my holdings - but not the stock that had landed in my account - at a loss, because being unable to daytrade the following day would be worse than that loss. (As compensation, they offered to refund the commission on that trade.)
Just like you don't withdraw money that erroneously appears in your bank account (something that happened to me when I was a kid, and no, I didn't withdraw the money), it seems unwise to sell stock if you know you didn't buy it, because at some point presumably the opening trade will be unwound and you will wind up being very short.