Correction, those keys are genuine(otherwise the MS license servers would never approve the install) but carry very strict TOS, meaning they're illegal to use for most people. The keys you find on such sites are usually dumps from MSDN accounts. These accounts are alloted to companies and carry the requirement that you may only use them to upgrade an already licensed Microsoft product. You can't legally use such a win 10 key on a system which never had a genuine win license. Also, the keys are meant only to be used by the alloted company, Microsoft will block keys it thinks violate these policies.
You're best bet is to buy directly from Microsoft.com:
Win 10 Home |
Win 10 Pro