The reason they work via PM is to bypass any seller/post count restrictions other forums may have and also so that other people viewing the thread can't see the 'stolen' pics they often use.
In order to allay any 'scam' fears, some sellers even offer to take a deposit and despatch the goods on trust that they will get the balance as the buyer is an established forum member !
Suggestions posted on other forums with regard to establishing seller's legitimacy include;
- Ask for pics with seller's name written on a sheet of paper in front of the goods
- Do not pay by Paypal gift
- Check how long the seller's been a forum member
- Search Google images for what you're looking to buy, to see if that's where the 'seller' pics came from
Few forum links about 'em (external sites, may contain some swearing)
First read it here;