daytrade diaries... july 13, page-52

PJ, I am also trading BLY. It is my first time to trade margins like you guys do. This was What I tried to do: I placed a buy order @25.5c and and monitored the market depth. I added vol of all the bids @25.5c and all asks @26c and make sure bids not less than asks(this way after I buy @25.5 I place a sell order immediately @26c and the sell order will be in a position that very likely to be filled). Otherwise I would cancel my buy order.
But the prob is, though generally market moves very slowly, it can suddenly go down(maybe suddenly go up too). Like the trade I did, in 5 seconds all the 25.5c bids gone , my order was executed and the share started trading @25.
What do you think I did wrong?