GOG’s EULA allows you to legally transfer any games as long as you destroy all not transferred copies.
How does that work with the licence grant in their systems? Is the burden of proof on you, or do they have a mechanism to transfer that?
It works on the honor system, which is just a fancy way of saying it doesn’t. Mainly because nobody wants it. Turns out reselling, as a concept, doesn’t really make sense when games go on sale all the time.