Customers are always responsible for the payment of customs/duties and any fee associated with delivery to their country. This is clearly outlined in the terms and conditions for buyers:
From Ebay: Import duties, taxes, and charges are not included in the item price or shipping cost. These charges are the buyer's responsibility. Please check with your country's customs office to determine what these additional costs might be prior to buying.
It is 100% on the customer. Not sure why it would be sent back at your expense? I've had unclaimed mail sent back before and they just return it the address recorded with no additional expense.
The only other thing I can think of is that Ebay has started charging buyers for their duties upfront when they order, so double check the transaction and see if there was any tax/duty added. If so, you can advise the buyer that it is already paid and they can advise customs (who would have to open it to see your paperwork), but the better way would be to enclose a copy of the invoice on the outside of the parcel in a ziploc bag that customs can check at a glance.
