I'm straying away from the topic here but this discussion probably explains why a lot of retail brokerages post your order on the inside bid or ask instead of hitting the inside bid or ask so that they can collect the rebates. To wit: If there is an inside bid at 32.00 for 100 shares and I place an order to sell 100 shares at 32.00 I've seen my order post on the inside offer at 32.00 instead of taking out the inside bid at 32.00. It has happened to me several times at my brokerage. I'm not saying this happens at all retail brokerages. It is something I have experienced not infrequently and I'm beginning to think that rebate collection on the brokerage's part is the reason. I know that commissions are what brokerages are after but if they can add on the rebates to the commission then that adds to their profitability throwing this thought out there fwiw.