But even that can be photoshopped, ultimately its a number of factors including length of time and number of posts, whether the member is active and interacting. Generally if it's a big ticket item then paypal goods and services, what you're paying for is insurance. Dont be cheap then complain!
Anyone who turns up 5 minutes ago, is only on the classifieds and has a good deal is probably a scammer. Some of the smarter ones might work on a profile for a while and even ask quite high prices but generally if you ask enough questions they fold. The obvious ones come from non western countries, but the real aszholes are locals who know what to say and are part of the Ducati community like that dude from NY who burned half the forum a few years ago