Heyyo, I got mine from Reddit in their Microsoft software swap subreddit. They've been fine. For OEM keys sometimes they don't validate so it's a matter of contacting who sold you the keys and getting a replacement one.
So the subreddit I mentioned is the same deal as KinGuin.
Kinguin and other grey marketplaces do very rarely sell keys that doesn't work but you dont know if the keys are stolen or not. But 99% of the time you at least get a working key