Shure they take a margin when they can, You are using limit buys so thats your price. Sometimes they take your order and lump it with a big sell. You got your shares and thats what count. If your broker charged you more, get a new broker. I've had bottom feeder buys go in when the price never got that low. They sometimes grab your order in order to fill something big. Doesn't happen all the time but its thier job to create a market for these things.