FABO Gold Members selling products
Other forums I'm a member on, prevents users without X amount of posts to be able to post classifieds. (25, 50, 100 posts depending on the site), unless they become paid members.
Strict rules on the classifieds there are enforced, even with watermarks for that forum, or edits on every image with the for forum address on it.
On those forums, Not following the rules results in the deletion of the classified posts. Bans after 3 deletions/warnings are also enforced.
Each forum's admins, mods and owners collectively decide on such rules and enforcement.