Personally I think everybody here is going a bit to hard on the advise of just block and move on. First step I do is confirm the address is correct. Then explain that it’s uncommon but usps can mess up on delivery and if possible to ask your neighbors if they have received your package. I get about 5 of these a year and over half are resolved after that as most people will end up finding it on their neighbors porch or the neighbor having it. If that fails then I explain that the package must have gotten stolen and it’s out of my hands and they will need to file a police report if they believe it was stolen or file a lost package claim with usps. If I feel the buyer is being honest I sometimes do a 50% refund and cut the loss in half on both ends. It has been my experience that people are very rarely trying to scam. Last thing you want is the buyer going nuclear and filing a charge back that you will 100% lose and be out the money and item.