The whole business with people selling product keys from scrapped machines on eBay is extremely dodgy and almost certainly violates the EULA. Given that Microsoft sets pricing, if someone is selling a product key for a large discount then there is certainly something dodgy going on. There's no way that anyone in their right mind should be buying them and TBH eBay needs to get better at stopping these scammers.
As far as the product key thing being embedded in the UEFI goes - You have never been permitted to transfer OEM licences to a new machine so whether it's paper or embedded it doesn't matter. The only real downside is if the board fails as the EULA does permit moving the key to replacement hardware to repair a fault. That said, it's only PCs from big manufacturers that have the key embedded, if you build it yourself with a purchased OEM licence then you'll still have a key you can use on the replacement hardware. If it's from a manufacturer and still covered by warranty then they will deal with it (When I've had it with Dell, the replacement board won't have a key in the UEFI and instead they will give you a "replacement product key" on a sticker). I suppose the only concern would be in the event of a motherboard failing out of warranty - Not sure what you can do in that case (although you could have potentially thought in advance and extracted the key from the firmware (yes, you can do that, there is still a key, it's just not on a sticker any more). Alternatively if you are buying replacement boards for an OEM machine, chances are you'll be looking at used ones which probably already have a key embedded. Of course, I don't actually know how the EULA holds in these cases.
I don't really have a problem with the embedded product key thing, sure it would be nice to get a paper copy as well but in the end, I've seen way more people "lose" their product keys due to stickers getting removed or worn off rather than motherboard failures.
