n0ns3ns3 :
some people/organisations are reselling their legit unused key that they go in some way - bundled with PC or whatever. they sell it at low prices.
I'd also sell a spare license.
It is legal to buy things with invoice, taxes etc. it's not a black/underground market.
If it's sold in a store and activates, I personally consider it legit enough.
And what MS (or any other corporation) thinks about those keys is not important or not my problem.
I doubt you never use ebay or questioning the origin of the products with significant discount in some supermarket.
You selling *one* license is one thing.
When a seller has 1500 of them, at that drastically reduced price....the reason is pretty obvious.
Kinguin is not a "store".
It is a meeting place between you and the seller.
And as far as activates? Yes, they may activate initially. Until the activation servers catch up, and recognizes that the same license has been used and activated 30 times, in different countries.
"It was fine until I reinstalled..."
"It was fine until the Fall update..."
"It was fine, until I woke up this morning.."
What MS things is actually relevant, because they can and do cause those licenses to unactivate.
http://www.tomshardware.com/answers/id-3304726/windows-key-working.html
http://www.tomshardware.com/forum/id-3475215/kinguin-windows-key-wtf.html
http://www.tomshardware.com/answers/id-3317697/windows-home-premium-key-typed-invalid-activation.html
Just a couple of the many we see here.