Responible according to §5 TMG 
VAT number in accordance with § 27 a Value Added Tax Act

The European Commission offers a platform responsible for online-dispute resolution (OS-Platform) that you can find here:
A list that includes contact details of approved dispute settlement bodies might be find here: 

Online-Streitbeilegung gemäß Art. 14 Abs. 1 ODR-VO:
Die Europäische Kommission stellt eine Plattform zur Online-Streitbeilegung (OS) bereit, die Sie unter finden.