Seeing as you're in the minority here thinking that, why do you say it's not?
Because they get keys illegally or from foreign countries where the keys are cheaper.
Also, it doesn't bode well for your business when one of the top suggested searches is regarding whether or not your business is actually ok to use:

I'm sure people get plenty of keys for cheaper than retail value that aren't revoked or aren't illegally obtained somewhere down the supply chain, but as for the question of is a grey-market website legit, I don't think you can say it is if there's a risk associated with buying keys from there. Personally, I'll stick with Steam, Humble, GMG, and gog to get my games, just seems simpler and less risky overall.