By "wait until it was a problem" I meant when the classifieds are overrun by brand new members trying to sell their stuff, and the ads belonging to members who have been around awhile are getting buried.
It does not matter much whether a person has 1 post or 20 as far as how much you should trust them when completing a transaction over the internet. Hence rule number one of the Classifieds Section: BUY AND SELL AT YOUR OWN RISK! As when buying anything online, use good judgment and caution.
While I would hate to hear about someone getting ripped off from another member here, I don't think requiring someone to have a small number of posts before they can post an ad would really solve that problem. If someone is selling a big ticket item and they only have one or two posts, take that into consideration before you send them any money. Most people wouldn't even consider sending money to someone who just joined a forum and is trying to sell something of a considerable value.