I'm sorry this happened to you.
If the post office says it was delivered, it has nothing to do with the eBay seller. It might be possible that the package is still at the post office. I would recommend physically going to the post office and talking to them.
Sometimes "delivered" just means that it reached the zip code. Sometimes "delivered" means that it was given to some other occupant of the address. Sometimes "delivered" means it was dropped off at the neighbor's. Sometimes "delivered" means that it was dropped on your doorstep, and some lowlife scumbag stole your package. Sometimes "delivered" means the buyer got the package, but misplaced it (though that doesn't seem to be the case here).
Either way, a seller can't do anything to magically show a non-delivered package as "delivered" in the USPS system. At the very least, I would recommend filing a claim with USPS for a lost or rifled package (whatever they're calling it these days). Sometimes the package will show up after that, though it's not likely. But it's worth a shot.