If its based on post count whats stopping someone from just posting all day to get the proper post count?
I think it needs to be a little more organized than that. Perhaps start with that, and create a list of posters with 2k+ posts or whatever, then after that to get on the "ok list to buy/sell" you have to be recommended by 2 - 5 of these established buyers or sellers after completing transactions with them. Of course, this will require a lot of effort and dedication of a few posters to really keep it organized... but hey just an idea.