I hear and acknowledge your grief.
I agree that eBay should be more amiable with sellers. I'd like them to be the selling 'partner' we expect them to be. But I digress.
So many things went wrong here that it's difficult to say any party is more wrong than the next. I mean, it was rather like a perfect storm of seller 'errors', ebay's policy and arbitration, and buyer's attitudes. And I can't say that your situation is common. It's a fluke, and one that went badly for you. I'm sorry to hear it. Really!
I would do as these other replies suggest and contact USPS, have them investigate the signature confirmation, you need to ask for documentation, and it may take several phone calls and emails. Then contact ebay with what you find, etc. I'd bother the hell out of them, and perhaps take as much as $900 of their time as consolation. Which will likely take a lot of effort as well. And you may not get anything back even then. But is it worth trying? You bet. I would.
There are several types of signature confirmations, and it probably didn't use a direct one (the name on the label), but just anyone at the residence could sign, an adult presumably.
There are some things I would have done differently to augment this outcome more in my favor, such as understanding that the customer who seeks a return for "not as described" is all but guaranteed a return. It's not fair, and perhaps even unethical, but that's ebay for you. It's imperfect and they should work on doing a much better job. Forcing eBay to step in isn't in the sellers interest, and costs you. It's not worth the gamble.
Yes there is a conflict of interest in having ebay be the abitor - of all things, and their decisions are final. And they're so efficient but terrible at it, like rubber stamping most customer claims. And - We know that, if we've been on this site for any length of time and had to deal with a customer opening a case for "not as described". It's a well know meme by now! A total joke. And while there may be times they do agree with the seller, they're few. And Most of the time, the buyer seems to be fishing for a refund (partial or full) And keeping the item. These sorts of buyers should be banned if they're found repeating this shenanigan, but I don't know that they are. Also, block a fussy or sus buyer. Nip it in the bud from having to deal with that person again.
So, the Best Management Practice for customer complaints is to (however grudgingly) accept their return request, quickly as possible, which limits the customer from elevating a case to ebay to arbitrary rule (usually in the customer's favors), and keeps the outcome a reliable one where the seller gets the item back, even if they're out the round trip shipping. This is the way.
I do hope you can somehow work this out. And I would say, in the future do limit your exposure to loss and grief by having a well thought out plan to mitigate situations like yours from occurring again.
