> Taken to the extreme a middleman exchange with infinite resells and instant transfers of ownership could limit total sales to peak concurrent users. I mean why should an American and a European each buy a license? They could just share a license and instantly transfer it back and forth when they want to play!

This can be mitigated by limiting transfer licenses by some days, i.e. after transferring you need to wait one month to transfer ownership again.

However, this wouldn't even be necessary. Just a process that is boring enough to do would avoid most non-malicious abuse. For example, to transfer the user that current has ownership of the key must went to a process of disowning the key first.

There is already better ways if you want to share your library with some trusted people: i.e. Steam's family share.