the hidden rule of paypal is follow the money. If the buyer pays via Mastercard you are generally ok, MC usually sides with the seller not the buyer on most online disputes. Visa is 50/50 and sort of depends on the bank. If the buyer paid with AMEX you are screwed, they will yank funds out of paypal accounts with no hesitation. I honestly don't know about payment with bank accounts which are like 90% of paypal funding, I guess maybe it depends on the bank or something? If the buyer paid with funds already in his paypal account its probably pretty safe, it would have to be very obvious fraud for paypal to reverse that.
basically paypal/ebay will always protect themselves over everyone else, so as long as they aren't the ones stuck holding the bill you will be ok. but if your buyer used stolen credit cards or ebay/paypal account, then the only person around who can be left holding the proverbial check is you... the seller.