OP: Sorry that this happened to you.
But don't lose faith. I have only been burned once in about 40 years. I think that it is a combination of luck and due diligence which has worked for me.
eBay is actually a good place to buy because if the item really is not as described, you generally have recourse and in most cases can return the item and get a refund. In fact, the flip side is that many dodgy buyers take advantage of same and screw legitimate sellers. They will buy, receive item, remove & swap needed parts in order to fix another unit, and then return the original item claiming that it was not as described. I no longer sell on eBay because it is too risky.
In your case, if the eBay seller listed the item as "Non-working, for parts only", then probably not much you can do even if they claimed otherwise in the description.
Another thing that has helped me is that I usually only buy from certain, known sellers. I also check feedback ratings and spend some time reading feedback even if its 100%. That helps me to form a gut feeling about the whole thing.
Again, I'm sorry. It really sux to take a hit like that. Here's to hoping that you get it resolved a come out with a working amplifier.