Contact your local post office, not the 800 number, but if you look up your post office location you can get the regular phone number from that. Tell them the issue and the tracking number and ask where the carrier was when it was scanned delivered. Is it a larger package? Is there a community box for packages and the mail carrier puts the key in your box? Maybe they put the key in the wrong box. Ask your neighbors if the accidentally received your package. My neighbor got a UPS box of mine and didn't know what to do so they put it in their basement for months. Didn't know anything about it when UPS came knocking on their door. Didn't know that was our name on the box, you would be surprised what non action people take if they get a misdelivered package.
Then if it was priority mail or another insured service, contact the seller and ask them to either file a claim or provide you with the information needed to file a claim. The seller has no obligation to you if it was scanned delivered, so be aware they don't owe you anything regarding eBay policies.
If it was an expensive item, you prob need to file a police report if you are going to file a claim on your renter's insurance.
