Like ebay, paypal has its own verrificaiton system. Retracting payment is always possible but highly unlikely and very hard if you are selling from a CONFIRMED accoutn a CONFIRMED account. When this suer sends you payment via paypal, paypal will tell you his status, confirmed or not. If he is not confirmed simply rject the payment and inform him that he does not meet your guideless for payment exception. Ebay has a long list of spescial rules for payment issues and questionable payment.