I know one way is to ask that the buyer checks "services" instead of "goods" or other tangible forms of a product since PayPal readily offers chargebacks for non-tangible products (since they are the most likely to be rip offs).

Otherwise your plain screwed, because as long as your customer has paid with a credit card through paypal they can chargeback as much as 4 years later I think (dont take my exact word, but its a long time).