I would agree checking the seller or buyer out is the best way. Some people unfortunately try to buy when they have not had any posts and that rings alarm bells.
I ended up trading my celestron 6se as I asked for bank deposit and not paypal. Those were the terms and the buyer accepted, so about the safest way to sell. As a buyer I get nervous about bank deposit because people can simply take your money and when I had that happen, the police took details but essentially did nothing (although they did cite the person was already known to them ...). Best to also check beyond the web site for names - googled the scammer I had and found he had a history going back some years. but too late
As for systems, they are often hard to implement and then what rules you implement may seem a good idea but then you find they restrict or do not cover other aspects. Also takes time and money to implement. Often they keep honest people honest and scammers work out a way.
I like the fact this is a free web site and have met some great people on here. If anything is done it needs to be simple but effective (harder said than done).
