People would post price drops like in the MM, it would create extra moderating too. Also what if a deal went bad? It's not tooooo bad in the MM when we have a problem to deal with, but when it's a car for a considerable amount of cash it could be a problem.
I personally don't think we need car sales on here but when a 'deal went bad' it's not your problem, just like its not autotraders problem or pistonheads problem either. Plus car purchases are conducted face to face which most MM transactions are not.