If you found a private buyer (as in not a gold site), you'll be able to in almost all cases get the gold back and get him banned; inform him. Act professional, even if it's your first time selling gold. Set the rules, go hard, don't make him think he can change your price, "pay half now half after" or stuff like that. If he starts bitching, say you got other buyers and he can go fuck himself (even if it's not true).
Don't chat ingame about it (if you found him ingame). Get to Skype, AIM, MSN.
About PayPal, if he opens a dispute you can just get the gold back. Try to somehow confirm he's the owner, by comparing his Skype name to PayPal name, prefer premium accounts. This is how I sell, I don't sell to dodgy people without premium accounts, since they usually have more to lose than a new created account/scammed account with a name that differs from their Skype. But this shouldn't happen if you inform him: make a dispute, you'll lose your gold, you'll get a ban, and your PayPal account will be partly locked!
