I've said it before and I'll say it again........ Rating system would cut the wheat from the chaff. If a vendor was constantly having bad ratings then the prices they achieve for their stock would reflect this thus management would have to change their business model with the trade side in order to get decent money from the buyers on there. I don't buy off there any more, fed up with liars and having to get train tickets back, when I walked away from a car that took me around 5 hours to get to (3/4 different trains etc) then I made the decision to cut ties. I've had the arguments with Ian (Hodges as BHM referred to him as) and you just get a load of bull from him, there's no enforcing their policy that Jack references in the post above at all yet they won't shut that vendor off either as it's an income stream.
Jason hit the nail on the head it would take a mass boycott for anything to be done but sadly this will never happen