it might be someone who got a hold of a pre production version, it could also just be a scam.
Now are you talking from experience or just repeating things you heard on the Internet? And an unlabeled "preproduction" item sale would certainly also be a scam, not just against the buyer but against the original maker who obviously has a stake in (if not a binding agreement for) getting their review copies back. The heavy use, scuffs, and general wear from transportation such items usually have would weigh heavily against the soundness of purchasing one for daily use (not to mention the potential problems explaining how you came to own a stolen piece of equipment) count against the neatness factor for having something that may differ from the production item (probably not in any meaningful way).
(For what it's worth I saw a EOS-3 on sale the other day that was claimed to be shown at PMA 2007 or some such, shame I didn't have money to burn as it was a nice example of the camera. And at least once I have bought a prototype item from eBay, though not from Canon, and it was indeed a prototype.)
Shopping a clearance house like Amazon is somewhat shaky ground, as they do not necessarily get their products direct from the OEM like B & H.
How then does Amazon get brand-new items, if not from the OEM? They have their own warehouses. The only concern I've had about Amazon is that listings might differ from one to the other - one might be for Amazon selling the item, and another might be for somebody else's sale. I haven't seen any indication that low-quality sellers are being let loose on Amazon to scam people, though. The only advice is the general "look at the actual listing to see what's included" and don't buy used unless you're willing to take just a bare, used item.