@beachwanderer
Thank you for the information and the links.
For those who want to have a closer look at the court decision discussed and some central principles of law (immaterial goods) involved:
Ruling of OLG Hamm 15. Mai 2014 · Az. 22 U 60/13 (German)
http://openjur.de/u/692344.html
https://de.wikipedia.org/wiki/Erschöpfungsgrundsatz
Most of it is way above my head. One line of argument for validating the clause prohibiting re-selling seems to be that the ebook "owner" must copy the book to sell it (like sending it via email to the buyer and AFTER THAT deleting the own file). Copying the content is illegal, so re-selling can't be possible
. The court sees the Terms Of Use clause even as
consumer-friendly it prohibiting to do something which would be illegal.
... Oh, come on ...