Who will monitor the monitors? Informal law enforcement and collusion at Champagne

► Studies monitors' vulnerability to collusion, focusing on the Champagne fairs. ► Private judges at these fairs had incentives to collude. ► I build a model with a collusion proof equilibrium, unlike Milgrom et al.'s model. ► Allows for competition between trade fairs, role of merchant guilds/communities, and network effects. ► Tries to capture the historical reality more closely.