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.