I actually think this is a great idea, with one simple modification - make it an anonymous poll that all members can vote on with three options - Cheap / Fair / Expensive
The only catch with this would be if the seller revised the price (and if they were gaming the system, revised it UP!) so you'd also have to prevent editing of posts in selling threads, and maybe also zero the votes if price changed, and, yeah.. starting to get a bit messy now...
We have a system already for people to check prices - just ask in the 'how much is it worth' thread to get other people's views . This avoids derailing someone's thread with implied criticism or praise of their pricing that might not be justified, but helps a newbie, or anyone actually, get an indication on value from others.