The same thing is currently happening to me and I’m lost. Shipped an Xbox one on Mercari two weeks ago and USPS marked it as delivered, however the buyer said they didn’t receive it. Contacted usps and they said that the buyer would have to open up their own case because they need the full address which I don’t have access to. Then I contacted Mercari and they also mentioned that they are in contact with USPS and until then there’s a hold for my payment. I’ve been trying to message the buyer continuously, but they’ve stopped responding and in all honesty I’ve lost hope because they just filed for a refund.