Just do away with posting in classified listings altogether. Potential buyers can PM the seller for questions or to buy items. This will also keep the seller from post dumping. Warn users in the classified section rules that making more than one classified for any given item will result in a ban & the ad being deleted. Removing the users ability to delete the classified will prevent deleting old ads to create new ones. Set it up so that classifieds are listed for 60 days then deleted. Require pictures with username & date for all classifieds. No exceptions: everyone posts pictures with username & date, whether you have 0 references or 100. It makes it easier to moderate & it's not hard to write you name on a piece of paper & snap a picture. All for sale ads must list item name, description, & price. Violation of ANY rule should result action as follows, & moderate consistently, absolutely no exceptions for anyone:
1st offense: 10 day ban
2nd offense: 30 day ban
3rd offense: IP ban