There was an attempt in Germany to sue Steam to allow users to resell their entire Steam account(1). However, that effort failed. I think they should have pursued a case that focused on allowing Steam users to resell individual games. There may still be a court case that could be won that focuses on reselling the individual games keys between users.
I think in the EU though there could be some debate about whether games are considered "software". Only software is covered by Oracle v UsedSoft as I recall.
(1) https://theundisciplined.com/2014/03/20/losing-steam-is-used...