[T206DK]

I sold two cards to someone off the BST board last week. I shipped them out to the appropriate address with delivery confirmation. Insurance was not requested by the buyer. He emailed today telling me he received an empty padded envelope in the mail and threatened me with " other action" if I didn't respond. I' have never had an envelope just show up empty, so this is anew one in my 15-20 years of selling through the USPS. I have the original receipt that shows I didn't mail just an empty envelope. This seems strange to me. Is there that much postal theft in New Jersey or points in between ? How would someone know there were 2 old cards in that envelope in order to conveniently steal them , then re-seal the envelope to make it look like it had never been opened. Any feedback would be appreciated. I was having such good experiences selling and buying on the BST , and now this Ebay-ish thing happens to me