We don't have much information to go on, but after this time it can only be a credit card chargeback - either by the buyer, or possibly by a third part if credit card fraud was involved.
As buyers now pay eBay chargebacks are now brought against eBay, which can claim them back from the seller. In some circumstances eBay indemnifies sellers againt this charge. I haven't time now to look up the conditions and I won't risk trying to explain them from memory - you can find them in eBay's money back guarantee policy (near the bottom, I think) see: https://www.ebay.co.uk/help/policies/ebay-money-back-guarantee-policy/ebay-money-back-guarantee?id=4...
If eBay doesn't indemnify you against this charge, for the money involved you may need to take legal advice.
