Second I did not touch the legal aspect, just stated what the EULA contains and that it might be enforced by technical means. The law has its differences all over the world, so let's not go down that rabbit hole.
Here is another (regional) EU court result concerning exactly OEM Microsoft licenses, Microsoft didnt appeal further.
https://www.usedsoft.com/assets/Law/usedSoft-PM-Urteil-LG-Ha...
http://www.guytechie.com/articles/2010/2/25/how-slp-and-slic...
That's not to say there aren't ways around that, but it might be on a different level than just selling an OEM license.
Dates back to at least XP with Windows. The string that was checked in XP was very simple and later versions were digital certificates. Windows 10 OEM (started with 8 iirc) use per machine fingerprinting and have individual keys.
https://en.wikipedia.org/wiki/System_Locked_Pre-installation
System builder licences are fairly cheap but the EULA for them state that they are for putting on a new machine and selling that hardware on, if your building a machine for yourself you need a standard licence.
As for the cheap licences found on sites such as Kinguin. Speculation is a number of keys on these sites come from stolen CC's. Buy keys from legit sites with stolen Cc, sell the codes on Kinguin/G2A. Legit store is hit with charge back once CC holder realise they had their card has been used. It's been a fairly regular complaint in the indie games industry http://tinybuild.com/g2a-sold-450k-worth-of-our-game-keys
http://curia.europa.eu/juris/document/document.jsf?text=&doc...