You must have gotten real lucky then because if somene actually sends someone the money while using a different IP-Address and then filed an Unauthorized Transaction dispute, the disputer will usually win, actually almost always 99% of the time.
You do have some hope though, if he didn't use a different IP and filed an unauthoried dispute, they can check this and see his account hasn't been hacked and that it was infact a legitimate transaction, you can call PayPal and tell them the buyer is just trying to get a free product out of you. Of course make it better than that but you know what I mean.
Good luck getting this resolved.