Since the asking price is not necessarily the selling price, why not insist that buyers post their offering prices as well. If the card is sold, it is reasonable to assume that the card sold between the two prices.
As a seller with multiple cards in one thread, I see nothing wrong with removing a sold card and the asking price when it is sold. It is difficult to resell a card that is already shipped
and believe it or not, some forum members only see the card and price (if it is there) and not the $OLD.
Excuse me, but I have to return to my closet to haggle now.