I know this isn't what you want to hear, so you may think I'm not being helpful, but in all honesty, you need to move on and stop wasting your time. The truth is that you bought from an unauthorized retailer and you're completely SOL.
Why do you think the "authorized retailers" usually have higher prices? The best deal is never through them, but they rely on more than price to entice their buyers. That's the way it goes. You can't possibly expect HP to refund you when you bought an item on Ebay from some random third party. Heck, on July 15th I had the ability to buy a ton of 16gb tPads for $399, why you paid $450 I have no idea. But then I could have put up my tPads on Ebay for $450 and you could have bought one from me.
At that point, I'm just a seller on Ebay, I have no more obligation to you for anything. You agreed to a contract when you bought and the seller came through on their end.
Did your seller say in their listing "no refunds/returns" ?? If no, why can't you return it? If that is the case, well, why on earth would you buy from someone that doesn't stand behind what their selling?
Good luck, but that's why I'd never buy this kind of product from Ebay.