I have a couple of things I would think about:
- How long has it been? Alert the post office again and definitely keep talking to them about it. During the Christmas season, Scans are not even close to trustworthy with this company. Case and point:
1. I had one customer whose item showed that it was never checked in, never moved anywhere, looked like I just printed the label and never sent it. 2 weeks after, it just shows up at his door. 1 day after - Scans showed up (but with bogus dates and times).
2. Had about 15+ packages this year that weren't scanned at the first two places, then just showed up at the house.
3. Had two packages show delivered - but weren't. They were delivered several days later.
There are more, but the point is that USPS is literally awful at the one thing they do - deliver packages. So I would be skeptical that it was ever actually delivered. They aren't losing 8 BIllion a year because they are good at it...
At the post office, ask them if you have any packages in the pile "postage due". I constantly have stuff sent to me that gets accidentally or legitimately put in that pile, then they never give me a card telling me, or it just shows delivered. Make them look - this has happened multiple times.
As for the Ebay part - ask the seller if he bought insurance on it. I generally always do if its a big purchase for cases like this (I normally send more expensive items with FEDEX anyway since they are a real shipping company). I wouldn't open a buyer protection case until you try to work it out with him. He is justified to think it may be a scam - I get a couple scam messages a week. I would suggest calling him - ask for his phone number. People are way different behind email.