An important point which you have not mentioned is whether this claim was made under eBay's money back guarantee, or as a result of a credit card chargeback.
If the claim was made by credit card chargeback it would have been made against eBay itself, and eBay would have been liable to refund the buyer. Under the terms of their user agreement, you would be required to reimburse eBay. Unfortunately you apparently didn't learn about this claim in time to ask eBay to contest it.
If it was a chargeback the big difficulty is that the claim was made against eBay itself it's entirely up to eBay whether to accept or challenge it. If they won't challenge it I doubt there is any way you can get the outcome altered. But you would still have to reimburse eBay as you accepted in the user agreement. If the buyer opened a case under eBay's money back guarantee there may be more scope to provide evidence that the payment was fraudulent.
Two other points are that unlike eBay's handling of payments, its money back guarantee is not regulated by the FCA and so not covered by the financial ombudsman. Credit card chargebacks are covered by the FCA, but the claim is between the credit card issuer and eBay. However unfair it may feel, you are not even a party to this claim and I doubt whether the ombudsman would be able to accept a complaint from you (although you could try).
I'm afraid that none of the above is good news. However, as you say, this is a complicated issue and beyond the day to day trading issues on which we try to help other members. It would certainly be worthwhile consulting Citizens Advice, but if eBay is continuing to pursue recovery of an alleged debt you need to see a solicitor. Good luck, you're going to need it!
