I wonder how these guys get their hands on all those keys, in their description it does say he is a Microsoft partner, and that he has an ID number but still... If you're saying OEM is tied to its original hardware how do these guys manage to get so many working keys, unless Microsoft doesn't care and actually hand them the keys themselves.
Pre Win 10, you or I could buy an "OEM" license. It would be tied to whatever hardware you installed it on first.
It was a little bit cheaper than a Retail license.
I can say I'm a partner with both Bill Gates and Jeff Bezos, and The Lord High Pubah of All That I Can See.
Doesn't mean it's true.
These cheap license keys, either OS, Office, or games, can come from a variety of methods.
Several channels of "Not For Resale". TechNet, MSDN, ex-corporate, EDU...
A Technet or MSDN subscription would give you personally dozens/hundreds of licenses, for your companies testing.
$400 for an MSDN subscription, sell 500 licenses for $20 each...profit.
Or, stolen credit cards.
Buy 1000 valid licenses, OEM or Retail, with a credit card that is not yours, and sell them cheap.
Later, when the eventual chargeback happens, they will get Unactivated.
The seller doesn't care, because he's gotten the money and has moved on to a new account. Wash, rinse, repeat.
Positive feedback on Amazon/Newegg? Faked.
Read here for some further info on these cheap licenses: