The VZBV, or Federation of German Consumer Organisations, has filed the lawsuit due to an article in Steam's EULA prohibiting class action lawsuits.
This article was added after a European Union ruling on the resale of digital content.
According to the ruling: "an author of software cannot oppose the resale of his 'used' licences allowing the use of his programs downloaded from the internet" since "exclusive right of distribution of a copy of a computer program covered by such a licence is exhausted on its first sale."
This ruling was advisory rather than legally enforceable, but Valve decided to be careful nonetheless. It currently has no plans to implement a feature for trading game licenses on Steam.
The VZBV also complains about the fact that anyone who declined to accept the new agreement was locked out of Steam, unable to access the games for which they had paid.
They have issued an ultimatum, stating that they will file a formal case if Valve does not respond by 26th September.