Sure. Here's the thing - we don't know the details of what was disclosed to the buyer in terms of the shipping. I am assuming that the seller listed that the package would be sent tracked and insured, and didn't mention the signature. If so, when the OP bought the item he accepted those terms, and based on what he has said he didn't specifically ask for a signature. Trust me some people are willing to roll the dice to save what amounts to practically nothing, no matter how much you try to convince them otherwise - not saying this is the case here, but it happens.
As far as I can see with the information we have been provided, the seller fulfilled the terms that were agreed to. If the OP can show that's not the case, I'll gladly revise my view on this. This is why eBay has denied a claim, and IMO rightly so.
You say you would charge back in a heartbeat - to me that's fraud.
Cheers, Al