Dude1 buys a license on Kinguin from dude2. Goes back on Kinguin and sells it 20x. Dude1 buys another license from dude3, sells it 20x. Dude3 buys a license from Dude1 (origin dude2) and sells it 20x. And this goes on and on and on hundreds of dudes reselling the same license at different times. You can literally find the exact same key being sold by different ppl. Microsoft knows all about this, but actually catching it in the act is only semi impossible, it does happen, the keys themselves being the issue as there are quintillions of possible combinations. Unfortunately, when Microsoft does catch it, all users of that particular key get invalidated, meaning you have only days to purchase a new valid key. If you paid insurance, (huh?) and you notify the seller, they'll just give you another key, sell you more insurance, and hope Microsoft doesn't shut that one down too.
It's called a gray-market, and to anyone actually honest, it's honestly illegal as all get out. You'll not find Kinguin or similar sites endorsed by anyone at Tom's, it's affiliates or its parent Purch Group.
Buy it legally, either through Microsoft, or Microsoft certified vendors.