Call eBay. This is a rare scenario. (though most international buyers who have not apprised themselves of their country's customs policy will state they've never heard of customs fees & taxes before)
This isn't something that every seller has dealt with.
I recommend requesting eBay's best practice (which is likely to wait for the return and refund, despite the fact the buyer's lack of information on the situation and unwillingness to cooperate will mean you are out the original shipping costs, unless you partial refund [up to 50% of the final sale total] and risk a negative, though it would likely be removed, under the circumstances, though no guarantee])
There is no benefit to further stress regarding their refusal. (though keep it in your pocket when avoiding eBay defects and less than positive feedback, through documentation) It is within their capacity to refuse and, as with domestic packages, there will occasionally be a refusal. We can't force buyers to do as they are instructed, though life would become a lot easier, if we could.
Best take is to just assume this particular buyer will never make the mistake of purchasing an international product that will incur taxes/fees again and consider it a win for the online selling community. One more down, millions to go. 🤷♂️
We've been selling internationally for 30 years. Just removed the UK and other countries for their updated liability customs policies. If they change them to be more seller friendly again, we'll readd those countries.
Hope this helps. 🙂
