Michael Dean said:I really like "Silk Road" by Expeditious Retreat Press. Not only do they have the rules for trade, they also have an incredibly huge list of trade goods that are good inspiration for new adventures. Plus it's a darn interesting read about the Silk Road.
Kax Tuglebend said:I've never had the chance to use it, but the Minrothad Guilds Gazetteer for BD&D had a good trading system akin to the Traveller system ~ its long out of print but only $4.95 from DTRPG

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.