First make sure that he pays with F&F payment. You can make sure about this by checking if there are fees or shipping address involved in payment info in your activity feed on paypal. If yes, then its not a F&F payment.
Second make sure that he puts this note when he sends money - '' I declare that I am the rightful owner of the account. I revoke all rights to this payment in terms of chargeback. I also revoke any right to dispute this payment, because I am paying Virtual Goods that I already received ''
And i think that paypal is not taking seriosly any evidence from skype / steam etc.
But you should try to make a deal with him trough email address from which he is sending money. Also record a trade that may help you. But the note i gave you is enough evidence, because with it he declared that he's only & rightful owner of the account so chargebacks due to unauthorized transaction is off. Second he revoked all rights to dispute payments as he paid for already received items. That should help, if he still issue a refund, just call paypal immediately, dont message with them or do nothing, call them and tell that you have a proof that you gave him account or whatever whats virtual item, he received and now he's trying to scamm you and you could also quote paypal from their rules that its not under buyers protection, give them some proof, for example your video and tell them that hes trying to abuse their protection.
Hope you understand :D