I will concede that eBay and PayPal continue to use the pandemic as an excuse to half-ass everything worse than they ever have, so getting that one customer support rep that knows exactly what they're doing often is a one in a million deal, but, as I posted earlier, I'm speaking from experience. I won a case years ago against a guy saying a PS2 game I sold to him was defective after having it for literally 30 days, which is the time eBay gives you to open a dispute. I won that case, then he went to PayPal and opened one there, which, I also won after a little struggle. Aside from the fact he had already opened a case on eBay and lost, I also provided PayPal a video of me play-testing the game for around 30mins, which was actually taken into consideration.
I know other people fortunate enough to have won INAD cases opened against them, and I've found it's more a case-by-case basis when it comes to PayPal, eBay not so much.
Anyway, droaa, if you intend to continue doing deals like this, I think for your best interest, you need to start setting some stipulations to your sales before proceeding with them, like giving the potential buyer a set amount of days for a return and such, things that you can actually include in the invoice, which PayPal will take into consideration. Never go with "all sales final, no returns", though, because that means jackshit. Also, remember that eBay doesn't own PayPal anymore, so they're not bound by arbitrary clauses anymore...although they do have their own.