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.
To me the Old School Renaissance is not about playing a particular set of rules in a particular way, the dungeon crawl. It is about going back to the roots of our hobby and seeing what we could do differently. What avenues were not explored because of the commercial and personal interests of the game designers of the time.
What are RPGs?
A game where the players play individual characters interacting with a setting with their actions adjudicated by a human referee.
Rules are an aide to help the referee adjudicate actions and to help the players interact with the setting.
Dice are used to inject uncertainty which make a tabletop RPG campaign more interesting than "Let's Pretend".
The only thing a player needs to do to roleplay a character is to act if he or she was really there in the setting in that situation.