Also, isn't there a law strictly forbids this in Germany? As a middle-man, you are not allowed to use their data to compete with them? Because the place I work has a marketplace with third party sellers as well and we have internal rules explicitly disallows this, cited by our compliance team because of the regulations.

Then it is good for Amazon that the European retail operations are run in Luxembourg under Amazon SARL I guess.

În Germany it might stand, but you are forgetting the American legal system is very much moneybased, so Amazon can just fight it in court an win, or at worst settle, which I have no doubt it's overall still a win