I think this would be an excellent feature. Doesn't need to be a rating system or anything complicated just another category in the forum marketplace and useful links section titled Feedback. Someone starts the thread "Feedback <username>" and uses a simple template like out of ten, would you buy again and comments. When someone wants to buy they would search for "feedback <username>" and if anything there can see. Next person wants to leave feedback they add to started thread. It would sort itself out and need little input from admin. A forum I'm on uses this system and works great.
