We test the importance of social norms for market interactions associated with negative real-world externalities in a large-scale experiment with a heterogeneous population sample from Germany. The majority of experimental participants refuses to trade, thus behaving in a moral way. Our data suggest the importance of norm conformity for the decision to trade as a significant share of buyers and sellers condition market entry on the decisions of others. Moreover, a majority of observers is willing to incur personal costs to sanction trading. Moral behavior is significantly linked to demographic characteristics and stated preferences and attitudes of the participants.