Is the rules as intended that sellers pay both toll (as per page 143) _and_ customs duty (as per 144)?
Let's categorize it as four different entities:
1) Foreign Buyer enters Market to buy from Foreign Seller who has entered Market. Buyer and Seller both pay toll, Seller pays custom duty.
2) Foreign Buyer enters Market to buy from Local Seller. Buyer pays toll, Seller pays custom duty.
3) Local Buyer buys from Foreign Seller who has entered Marketer. Seller pays toll, Seller pays custom duty.
4) Local Buyer buys from Local Seller. Neither Buyer nor Seller pays tolls or duty.
Adventurers selling goods they've gotten in other markets or on adventures are virtually always in group 3, so they will pay the toll and customs duty.
Adventurers buying goods for sale elsewhere will be in group 1. They'll pay customs duty when they sell later.