I'm not really a fan of feedback systems and reviews as it's easy for someone to leave negativity on both for trivial things, I'm working on a platform now which has a system built in as follows, a car described as clean needs less than £100 refurb, average less than £300, below less than £500 anything over £500 then don't bother using the system. I also give a guide on how to work out your cars condition with examples and prices. Anyone using this system knows the standards before using it. For instance £100 for a panel repaint with a light repair so if you pay £200 you adjust your offer accordingly. Eg if you'll give book for a clean car you'd set your offer to £100 behind book. Now if the deal doesn't go through because of a mis understanding I.e the seller missed something and a financial adjustment isn't made then a black mark goes against them. We will try and work with that dealer to improve their standards, however, if they've completely tried to pull the wool over someone then the buying dealer blocks them which goes down as a final warning another block and they're out.
therefore putting the onus on me to uphold the quality of people using it. Let's hope other platforms can introduce something similar.