Thread: New members

I've seen it done on other forums where they have automated requirements to post into a buy and sell. The best one I've seen is:
1) Member for 1 month
2) Has started a thread in the "New Members" Section
3) Has a post count of 5

It is by no means perfect, but it simply rejects any attempts to create or respond to an ad in the buy and sell until they meet these conditions, and when it rejects them it tells them what they have to do in order to get access. Almost no one ever reads forum rules if they're just told to, but they do learn and follow rules when doing so will give them something they want.