The GPL explicitly allows selling software under it, as long as the source is included (or a written offer to provide it or whatever the other anachronistic options are). It is pretty deceptive though to not indicate that what you're selling is not the actual product, but the service of providing it on a physical medium (in this case).
Sorry that i'm not helpful, but it may help to ask them to deliver the source code as well, as per license (did they even include the license?). Also, ouch, MediaMarkt is a respected thingy, not your random anonymous seller. Kinda shocked.