There are no legitimate Windows licenses that only cost 10€. They may activate but successful activation does not imply that you are not breaching Microsofts license agreement and they may decide to deactivate those licenses at any time
You can get £20 books for 50p, because either the person has finished using the book, and there are lots of books; or because the person got the book free (as a gift generally) and didn't really want it; or because they bought a copy and got a second copy gifted, etc.
Furthermore, this is not one vendor selling one ‘stolen’ item, it is many, selling many keys. This makes it seems like a legitimate channel for keys to the average consumer. It also makes eBay more responsible if you ask me. It seems to me as if they are making a profit from those sales. If they truly are illegal keys then they should probably do something about it.
So why am I wrong here?