At Licenceking I can also buy very cheap used licenses. I think it's good, but is it legal?

Of course it is. The European Court of Justice has already decided this in 2012 (if you want to have a look: ECJ C-128/11). In order to be legally on the safe side when buying used software licenses, it is essential that you buy exclusively from reputable dealers such as Licenseking. Here you will find further information on how and why you can legally save up to 50 percent (compared to the manufacturer's original price) with Licenceking.