I'd strongly advise against buying Windows keys from non-official sources. Not only does it often violate Microsoft's terms, but there's also a high risk of ending up with a counterfeit or already used key. If the price seems too superb to be true, it probably is.
For peace of mind and guaranteed updates and support from Microsoft, stick to purchasing directly from them or through authorized retailers. This ensures you won't face any activation issues down the line or potentially expose your system to security risks associated with fraudulent keys. Always prioritize legality and security over saving a few bucks—it'll pay off in the long run.