Just want to start by saying that it's only for a $17 transaction, so financially it isn't a big deal. I'm more concerned about letting a potentially sketchy buyer go unchecked.

Had a chargeback opened yesterday, for a transaction at the end of December (as I said, $17). "The buyer stated that they didn't authorise the purchase".

So I do a quick transaction history check and find that the same buyer has purchased from me prior to December - in 2015 and 2016, in fact - with a different account, but with the same name, address, and the username almost the same as the chargeback username (the only difference being a full stop).

Not knowing exactly what the buyer is claiming (eg: whether or not they're claiming that their eBay was also hacked, and so forth) I messaged the buyer.

To summarise, my message was, "I see you've filed a chargeback. I can also see that I have previous transactions with your name, address, and similar username from 2015/2016, so I'm confused. Please advise, otherwise I'll need to send all relevant information, including the above, to Paypal regarding this case."

This was a bluff: I have no intention of contesting the case because I don't have tracking and I am not paying a $15 fee. 

Buyer replies and says that he let his friend use his eBay  and Paypal accounts to make purchases, and that his friend put his card on to his (the buyer's) Paypal. The buyer said that he didn't realise that his friend's card was set as default payment method on the Paypal account still, and that he had inadvertently made purchases and used his friend's card to pay. He said he was "sorting it out" with his friend.

Hmmm. So if this is true, then technically it IS an unauthorised transaction ...

How would you proceed?