I have traded on here a few times and only had one person try and rip me off. They tried and failed. The moderators were great, paypal got my funds back and hopefully he broke his face on a curb.
Something that does bug me is the introduction of "buyer pays fees". I would much rather sellers factored that into their prices.
The main reason this bugs me is some people lack the math skill to calculate this and ask daft money.
Another thing is over inflated postage. Me personally, on large items I sell I over charge on postage to ensure i don't loose out and then refund the change back to the buyer.