I had something like that happen with ebay. Buyer paid, I sent, but then the USPS Tracking just stopped tracking. I refunded money. Weeks later I happen to check on status and it said it was “mis-sent” and is now on the way. Delivered a few days later.

I contacted ebay, and they said that its no longer a ebay transaction, I will have to work it out privately with the buyer. Luckily the guy was honest and sent the money owed.