Your only risk is when you sell virtual items through Paypal, because when you post a tangible good, such as a game, you obtain proof of posting from the postal service you used. So if he does try to claim his money back you can provide the proof of posting and he'll fail in doing so.
People can claim that you never sent the RSGP or something because it's virtual, and there's no way of Paypal knowing whether he did indeed get it or not, which is why it's not too wise to sell virtual goods via Paypal unless someone is extremely trusted or if its someone personal to you.
I know a bit about it because I have lots of experience with dealing with Paypal/Ebay/RSGP Selling and real goods selling ^^
Thanks