Check their Itrader before buying from them. Someone has two posts and no reputation that says "I'll send you 50,000 hits to your site for 5 bucks", your most likely just giving him 5 bucks for asking instead of buying.
The internet is like a scammers playground. You need to realize, that not only on DP are you gonna get scammed, but lots of places. You can't stop international scams, but you can do your part in preventing them by using the iTrader system. Of course probabally the most fraud is commited through loans and internet bank trading. EX: Poster A: I need to borrow $300 asap, can someone lend it to me? I will pay $320 in return. Poster B: I am able to lend you $300 Then poster A runs off with the money and doesn't pay you back. You file a dispute with Paypal or whatever, and its automatically your word VS theirs. If they don't have the posts or iTrader to back it up, simply don't bother.