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...
Oracle sued alleging copyright violation. The ECJ ruled that what UsedSoft was doing was not a copyright violation.
Applying that to Steam, it would imply that if I sold someone my Steam key for a game I had purchased I would not be violating copyright. It does not necessarily imply that Steam has to make or allow the key to work for the buyer. That's out of scope for copyright law.
"On 3 July 2012, the ECJ handed down its landmark decision in UsedSoft GmbH v Oracle International Corp (C-128/11), ruling that the owner of copyright in software cannot prevent a perpetual licensee who has downloaded the software from the internet from selling his ‘used’ licence. This decision has significant implications for the software and other digital industries." [1]
[1] http://www.linklaters.com/Insights/Publication1403Newsletter...
For example many countries started to require that mobile phone numbers should be transferable between carriers and mandated the carriers to build an infrastructure for this.