The only way fnf is refundable is if sender (or real PayPal account owner) files unauthorized activity dispute through PayPal or a chargeback outside of PayPal from sender’s card issuer or bank. Fnf doesn’t 100% protect you from everything. PayPal not going to help you since you sold something with fnf. Send an PayPal invoice next time or refund the fnf payment and insist on goods and service payment on sales. Scammers can prey on the fact that there is no PayPal buyer protection on fnf to give recipients a false sense of security but knowing they can get you on the chargeback.
Many accounts don’t give you right to sell or to transfer the account. Should really read the terms of service of the accounts you’re selling, too.
You’re going to have to add money to your PayPal account or any future payments will go towards zeroing the negative balance. If you do not, your account could get suspended and the debt sold to debt collector.
Kudos & Solved are greatly appreciated. 🙂