Sent packages have not been received?

Hey all!

I recently sent out four packages that were Pokemon cards in Topps loaders that I sold through Mercari. They were dropped at the Post Office on February 25th and still haven't been delivered according to the buyers.

I've sold cards with Mercari in the past and never had any issues until now. I'm going to call my local post office this morning to see if they can give me any information on the packages, but was curious if anyone else was having any issues?

I live in Kentucky. Mercari says that USPS doesn't provide tracking information for letters and to check back with the buyer, which is new to me, as when I was previously selling through Mercari it always provided tracking. What happens if my packages aren't delivered or if the buyers claim they never received the packages? Since there is no way for me to track them do I just lose the items and the money they are worth?