I have never gotten begative feedback from one of these experiences on eBay. If the winner of the auction requests that I send it to their sick son in Nigeris (or whatever the newest excuse is) I report it straight to eBay. They are pretty knowledgable about this type of scam and will shut it down quickly. They also gave me a refund on my auction costs.
I have gotten two scammers on eBay kicked off like this in the past month!!
Oh, and about what Nadz said. If you require that a seller have a Paypal account, that means that you cannot offer any other type of acceptable payment. And lots of (ok, well some) legit people like to send in cashier's checks and the such.
You will lower the demographic of your buyer's market if you only offer Paypal. 