Many of the market scams we have seen rely on the fact that someone that is making a bid doesn't need to have the funds to back it up. We used to have a large penalty for anyone caught doing that, but players complained about this. Now what I would like to see is to have the money actually put in a special frozen account while the bid is active. This would be symmetrical to the asks, where the stuff offered is also taken away from the seller's custody while the order is active. I realize that this limits the possibilities for risk taking that some traders might do (basically trading on margin), but overall I think most people do have the funds necessary when putting out orders.