The OEM keys that you can buy from Kinguin and twed are legit. They are just left over OEM keys, after Microsoft banned selling new computers with Win 7/8.1
HP for example, probably bought 1.5 million Windows 7 OEM keys from Microsoft. Then Microsoft (who desperately wanted Windows 10 to sell) told the various OEM companies, that they could not sell anymore Windows 7/8.1 NEW computers.
So, to make the best out of a bad thing...companies like HP and Dell...sold all their leftover OEM keys to Kinguin and twed
Thus, HP got some of their money back, and didn't break any Microsoft rules. At least that's how I see it.
The keys from Kinguin and twed are legit and do pass Microsoft's activation...etc.