Those are digital (unused) licenses, that although can't technically be resold, they "can" be in Europe as they contradict a ruling made in 2012 (I don't have time to dig the link up, unfortunately).

Those cheap licenses are always used. They exploit the fact that these can be used to activate ~10 copies of Windows. It's even worse than that though. One seller could keep track of this limit, but what happens is that a bunch of sellers source their keys from other similar sellers. So they don't even know how many times a key has been used.

Last time I bought one of these 15€ keys for a friend, I had to write to the customer support over 10 times and shuffle through at least 6 different keys until one actually worked.

To be fair they aren't necessarily unused. I made the mistake of buying one once and had to jump through a lot of hoops to get Microsoft to activate it. I'm still not sure that it was entirely legal, if I'm honest.