I assume that the international fees you refer to will be an exchange rate conversion to refund the buyer in his own currency. (I've never thought about this before, but as all refunds are now made through eBay I imagine that eBay has to pay this, and passes it on to the seller.)
You don't have much alternative except to agree to this cancellation request. You listed the item for international delivery, including South Korea, so the buyer has the same right to request a cancellation as if they were in the UK. As a private seller you don't have to accept - but to refuse would be an absurdly high risk. You would then have to deliver the item to Korea, knowing that the buyer can and almost certainly will "discover" some reason why it is not as described. eBay would then require you to refund the whole payment - and to pay for and arrange it's return. If you don't, they would simply enforce the refund and let the buyer keep the card.
There are risks at every step - including the real possibility that the card you get back might not even be the one you sold. eBay never sees the item concerned, and where it's the buyer's word against the seller's eBay admits that they can't know "who has the valid case". Unfortunately for sellers, eBay's default policy in this situation seems to be to take the buyer's word, and enforce the refund.
