Song China made legal ports worth using so merchants used them without being forced to

Song China's coastline stretched for thousands of miles, and foreign merchant ships could in principle land and trade almost anywhere along it, making direct enforcement against smuggling and unregulated trade essentially impossible for a state with no way to patrol every cove. Simply banning unlicensed trade by decree, without a practical way to catch violators, would have produced a rule everyone quietly ignored.

Foreign merchants also had genuine practical reasons to avoid unregulated trade if a better option existed: unclear legal status for their cargo, no standard way to resolve a contract dispute, and no protection if local officials or rivals simply seized their goods. A ban with no upside for compliance and only downside risk for detection would have pushed trade further underground, not into the open.

Starting in 971 at Guangzhou, and expanded to ports including Hangzhou, Mingzhou, Quanzhou and Wenzhou, the Song state established shibosi (市舶司), official maritime trade offices, as licensed ports of entry: foreign ships needed a permit from a shibosi office to sail and had to return to their original port of departure to register on return. 7% — in exchange for standardized processing and the protection of operating within a recognized legal channel.

Made legal ports worth using by adding standardized processing, dispute resolution, and cargo protection, so merchants chose compliance for self-interest, not fear.

Legal channel offered standardized processing and legal protection, which unregulated trade lacked.

Merchants chose compliance because it served their interests, not from fear of punishment.

State didn't need to catch every violator; the value proposition made compliance self-enforcing.

Customs revenue grew as merchants voluntarily used licensed ports.

To bring informal activity into a legal channel you can't police, make compliance the profitable choice by attaching real benefits, not just penalties.

Shibosi customs revenue grew substantially over the dynasty, from roughly 500,000 strings of cash a year under Emperor Renzong in the early-to-mid Northern Song to over 1.1 million strings a year at the Guangzhou office alone by 1140, representing an estimated 4-5% of total state fiscal revenue from that one office; other Song-era and later accounts describe the shibosi system's overall contribution to Southern Song state income as considerably larger, though without a precisely documented year or calculation.