Possibly raise the post minimum to 50 or 100 before anyone can post in the classifieds. They’ll just give up before posting that many times. Make it something they have to accept before posting a classified thread so they don’t post threads on how to get their classified ad in (kinda how noobs now create a thread for ‘why can’t I post’). Delete their classified thread if they don’t follow the rules, or post stuff for sale in other parts of the forum.
Keep the 5 post limit for regular posting, though.
Just my .02