Actually, looking back on this conversation, I still would like a response to this particular question:
"To make it clear: does Larian disputes this (first sales doctrine) right, and is it in their terms of service that one can not resell it (a game of theirs)?"
It's already been established by the US and EU courts one can (re)sell a good, including a digital one. So it would actually be against the law to forbid to (re)sell a game, which in turn means such a condition in the 'terms of service' would be invalid and not comply to the law in these jurisdictions.
In don't know the policy of Larian in this respect, but it's clear Steam and GOG prohibit it, which means they're demanding conditions that are legally invalid and are acting outside the law, at least in EU and US jurisdictions.
