Have you contacted the buyer? You need to do that first. Tell them about the mix up and ask them to return the payment. If they don't respond within a few days, give them one more chance, these days they may be affected by COVID. This time you indicate to them, that if they do not refund the money, it is considered internet fraud and you will involve ebay, US postal inspector, you will report them to their local police department and also report them to their State Consumer Protection Offices. Urge them to respond promptly indicating that it is a very serious matter.
If they don't then you need all the things above to get your money back. After each step, send the buyer a message about the steps you undertook. Don't make or accept any phone calls. Do everything thru ebay messages. Be professional no personal attacks, eg. you are a scammer etc.
