It required merchant barges or ships to unload their goods at the port and to display them for sale for a certain period, often three days.
Only after that option had been given to local customers were traders allowed to reload their cargo and travel onwards with the remaining unsold freight.
A related system existed in medieval and Tudor England, covering the sale and export of wool and leather and known as the Staple.
That affected the transport especially of perishable goods like foodstuffs, but traders could often pay a fee to avoid having to display their wares, thus turning the staple right into a form of trade taxation, with similar but less severe results.
Those advantages were not common outside the tollways, as medieval local governments were usually weak, and street robberies were frequent.