Is the sale and purchase of used software licenses legal?

Yes, trade-in second hand software, which first made its way onto the market in the EU or, respectively, in the European Economic Area, with the approval of the creator (thus the software publisher), is legal.

The Court of Justice of the European Union decided this in its decision dated 3.7.2012. In July of 2013, the German Federal Supreme Court of Justice confirmed the legality of the trade in used software licenses.

You can find all relevant decisions by the Court of Justice of the European Union and the German Federal Supreme Court of Justice regarding the legal trade in used software licenses HERE.