I just got a margin account at IB and am using their TWS java client for trading. I wanted to buy 400 shares of Motorola at 20.60 but I guess I must've clicked an 'Ask price' cell without realizing it and ended up 'selling' 400 shares at 20.60, i.e. having a position of -400. When the price dropped to 20.55 a couple of minutes later I 'bought' back the 400. I think I might've made a quick $20 (minus commissions) on the trade but I don't really understand what I just did. Did I just do a 'put' or what? Thanks.