Legit in the sense they'll work.... for a while*? Possibly.
Legit in the sense they comply with MS's EULA..... no.
Those keys could have come from numerous sources - bulk licensing not intended to be sold to consumers, education keys etc.......... sometimes they're legit purchases, that were made with stolen credit cards & are now being sold on.
*Depending on the route source, even if keys work initially, there's no guarantee they;ll work long-term and have the potential to be blacklisted.
Should you buy from them? That's entirely your call, but I wouldn't recommend it.
Regardless of what you do as far as a key goes, ensure you download your installation media from MS directly:
https/www.microsoft.com/en-ca/software-download/windows10
Any version of Windows &/or software from a third party site is near guaranteed to contain additional files (which could be adware, spyware, keyloggers etc)