Yeah, they aren't going to be keys that are allowed to be sold. They will more than likely work but the keys will have been acquired from something like MSDN or OEM keys ripped off of scrapped machines. By buying these, you aren't funding Microsoft, you're just giving money to the seller. The copy of Windows will most likely activate and work but it's no better than running a pirated copy.
The key thing to remember is that for Windows licences, Microsoft sets the pricing. If someone is selling them for huge discounts then the seller is either being very generous and selling them at a huge loss or are selling dodgy keys. You can also often spot in the fine print of the listings nonsense along the lines of "This is a genuine key recovered from a scap PC. As per eBay's terms and conditions, you may collect the scrap PC at your expense" - Even with that nonsense disclaimer talking about a PC that almost certainly doesn't exist, the key is still not permitted to install on any other machine.
