I think a great feature would be if you can ask a question to something advertised in the classifieds and then the seller writes a respone for all to see. It would eliminate having to answer the same question over and over but even more when there are great deals being advertised other users that may have a good reputation could write a comment for the newbies in the sport saying things like this is a fantastic deal etc? Or telling newbies to stay away from it.
I guess the downside would be it would need to be moderated but it could be a very attractive system and it could also eliminate people who do not check the forums from getting ripped off. Again though to keep it fair it would need to be moderated.