item delivered but unpaid
But it was paid! It has nothing to do with eBay if you accidentally refunded the payment.
Sadly, we know from many similar cases that all too many buyers will take advantage of any mistake to keep something for nothing.
It's up to you what you do, but I doubt you will ever get your money back. If thinking about the small claims court, take legal advice first. The courts are for breach of contract, but it doesn't appear that there was any binding agreement or contract between you with regard to this repayment. A breach of trust is another matter altogether, so if it was just a mistake with no obligation on the buyer to return it you may just have ro rely on their honesty. Or not. Good luck anyway!