Well, someone's funding source for his verified Paypal account could be a credit card. You, as the seller, won't know this unless he/she files a chargeback and even then it might be too late.

Withdrawing the money out of your Paypal account sounds like a plausible way to protect yourself but Paypal can still deduct the amount from your balance, leaving you with a negative total. It's happened to me before - not a pleasant experience.