Buyers can't cancel a purchase after it has been posted.
This is the UK policy 'If you need to cancel an order, submit the cancellation request as quickly as possible. If the seller has already sent your order, they won't be able to cancel it.'
Send your buyer the link:
https://www.ebay.co.uk/help/buying/buyers-can-cancel-order/buyers-can-cancel-order?id=4004&st=3&pos=...
I don't know if business sellers have to accept returns from international buyers? You could tell your buyer they can open a return after they receive their item and send it back tracked at their expense.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)