Not really sure there is a great way around this other than the old 'buyer beware'.
If you make it so that you have to reach a certain number of posts before you can post an advert then the scammer will just do that. If you make it too high then the classifieds will be dead - for example, 15k members have not even reached 10 posts!
If you introduce a feedback scheme (assuming it is even possible) you can then fake that - open multiple accounts and +1 each other. So I can't see how that helps and perhaps even provides a false view of trust!
If you look at this thread
http://www.golfgtiforum.co.uk/index.php?topic=144524.0 and I don't mean to pick on the last poster exactly but even after people have said in the thread it is a scammer, plus the signature on the scammer account, a user is still asking about the 'sale'.
Hiding the classifieds until you reach 10 posts for example, will mean that a very limited audience can then see the items for sale.
No easy answer!