Integrating commerce into your D&D game

The best merchant system for any edition of D&D I've seen is A Magical Society: Silk Road by Expeditious Retreat Press. It is exhaustive in its treatment of trade and trade goods. The best seafaring system I've seen is the Pilots' Almanac by Columbia Games. While for Hârnmaster, it is really a sub-system of its own that can be adapted for d20 use by remembering 5% = +1 on a d20. Silk Road can be found on RPGNow.