Dark Sun is a setting that is a perfect fit for such a fantasy milieu. The City of Tyr is a trade city with a clash of cultures, especially after it gaining its new-found freedom from the rule of its sorcerer king. The Dune Trader accessory (for AD&D 2nd) gives the rules for merchants and trading. The adventures are epic but still provide enough opportunities and motivation for trading characters (I myself played through the adventure path with a dune trader character, and it was much fun.)
Another good setting is Earthdawn. Here, the cities of Bartertown and Vivaine are good choices to center a trading campaign upon. I don't remember Earthdawn having specific rules for trading, but quite a few character archetypes and skills support the idea.