Is it permissible to buy and sell veal that has not been slaughtered according to Islamic law—and is available in German stores—to Germans, knowing that we will not eat from it?

If you know that the meat has not been slaughtered Islamically (ghair mudhakka), it is not permissible to buy or sell it. You must seek out a permissible alternative. It is not permissible to sell it even to non-Muslims, for it is unanimously agreed that it is not valid to sell carrion (maytah).