Kinguin (the site) sells nothing, and takes no responsibility for what's sold on it, so you'll get anything, and Kinguin turns a blind eye, they just do not want to know any details of origin, so really are blameless in this.
That said, when it comes to MS keys, that's shady as all get out. You'll get 1 person, buys a $20 key, sells 10 copies of the same key for $20, profit $180 And there's 100 ppl doing that at the same time. 1st dude then buys a $20 key from someone else, and sells 10 more copies, profit $360. And does it again, profit $540, going up $180 each time, rotating those 100 keys, total profit $18,000, actual expenditure of owner cash = $20. If a key doesn't work, you complain, seller just gives you another key, from a previous sale. The whole premise being there's so many billion keys floating around, MS will be taking its sweet time actually stomping on your particular key, but it does happen and you'll have 10 days to get a new one before windows becomes bunk. Legal? Not in any way, totally prosecutable if MS catches you selling, but those that do are hidden behind so much security that it'd take the CIA to break it fast enough to trace the original link.
Will I endorse Kinguin or other sites like it? Not a chance, that's tantamount to enabling software piracy, you are buying stolen goods. It's as simple as that. If a retailer wants to discount valid keys, that's their business, but those ppl on Kinguin are not verified/certified resellers or retailers, they are private individuals, and Microsoft EULA specifically states that MS owns the keys, private individuals cannot sell keys unless part of a working, installed software package on a pc.
It's illegal. Period. Buy the key from a verified/certified retailer, like Amazon etc, or buy direct from MS.