i think a feedback forum would be best now. Ratings are great and all, but its user feedback that's appricated the most. Each user must request to have their name there before they sell (should get rid of bots) and there will be positive sub forum and negative sub forum uder each user name, each forum closed. These should be "end deal" posts. The amount of posts alone in each should be a good "rating". Resolution should happen in PMs, email, and over the phone. WHen a deal is completed, based on everything, the buyer and seller should PM the mod, ask to be allowed to open up either the postive of negative sub forum, and then post it there, kind of liek waht they do with reviews, where reviews are posted, but a mod looks it over before it ever becomes a thread. It'll bring this up in the rating systems post, cause this might work out to everyone's benifit.
And yes, i know heatware, but an intraforum system is the best solution.