You know, I thought the same thing which is why I started this thread with the title I did. Never having purchased modern men's clothing off eBay I had no idea whether cheap knock offs were prevalent or not. I don't yet have the Bill Blass jacket in hand (it's due to arrive this Thursday), but the JoS A Bank jacket is beautifully crafted.
As another badger suggested I may just see which fits best and either send the other back if the seller is willing to refund me or simply pass it along to someone else for what I paid for it. Might even consider a trade for another 42S blazer/sport jacket of a different color.