I believe I have a buyer who is trying to scam me.

They purchased a $250 item from me, I shipped it to them via USPS Priority with Delivery and Signature Confirmation. The package weighed 4 pounds.

It was delivered 3 days later. 4 days after they signed for the item and received it, they contacted me said they received the package, but there was no item inside. I wrote them back and told them I can guarantee that the item was shipped inside that package, I packaged it and taped it myself, very securely.

I have not heard anything back from them in over a week, and they have not contacted yet or opened a claim or anything. Today I received my package back that I shipped them in the mail via USPS. Of course it was just an empty box that weighed less than 1 pound and there was no item inside. They paid for this empty box to be shipped back to me. The package has not been damaged in any way, and the tape was clearly cut when it was opened, not damaged or ripped off by a machine in transit.

Wouldn't this buyer have realized before he signed for the package that it was just an empty box and weighed nothing when the item he bought clearly should have weight to it?