A fine idea, with few suggestions :
-- Initially it should be in major cities.
-- PW members have a tendency to demand high when selling and bid low when buying. So this system might work out but not initially it will take some time
-- There used to be Jumma and Itwar bazars in Pindi/Isd but these were closed down by local auth due to traffic violations, frauds in deals, etc. So a system is required to check on these.
-- If at all this works out and becomes a hit then your dealer mafia will step in to stop the show
the idea is not bad but you have to be ready for anything.
If I want to sell a car and there is a such like place I'd surely try my luck