As a seller, this has happened to me several times where the package just disappeared off the face of the earth.
That is why I insure most cards that I ship. I don't ask the buyer whether they want insurance or not, I just insure it to protect myself.
After settling with a buyer, I always ask them to notify me in the event that the package arrives after they collect the insurance claim. As of today, I have never heard from a buyer stating that.
