You haven't told us why this happened.
If you just trusted the buyer to return the item when you had refunded them, you won't do this next time. There is nothing you can do about it now.
If this was a money back guarantee case through eBay's resolution centre, and you didn't respond to it or didn't send the buyer a prepaid return label, eBay will have enforced a refund and won't require them to return the item.
