A lot of the products by Expeditious Retreat Press in the Magical Society line, especially A Magical Society: Silk Road might be helpful here. Dynasties and Demagogues (AEG or Atlas Games, can’t recall which) might be a useful resource for developing political interactions and different social structures.
The biggest question I have is what kind of a merchant campaign do you want to run? Do you want it to focus on the action of exploring the frontier and taking on threats violently? Getting into the intricacies of trade disputes? Playing up political and social intrigue? Some combination?
As for overall advice, I've never ran a merchant/trade campaign. So most of what I speculate on how I would run such a game is just that, speculation. But here's some ideas.
Now, a lot of the usual D&D tropes applicable to other D&D campaigns you could easily apply to a merchant campaign. Bandits and other marauders are a typical trope; have them come up by threatening your trade routes and caravans. Thieves' guilds may want to get cuts of any trade transactions, or might enact protection rackets. Storms and disasters (both natural and unnatural) can give reason to explore the unknown for new trade routes. And crypts, ruins, dungeons, and caves may hold potentially valuable information, natural resources or artifacts to sell.
As far as any trade campaign goes, I'd think there are two big elements here: social interaction and exploration. Trade is a social endeavor, so having some party interaction and perhaps intrigue is almost a given. Who wants want, and who has the power to fulfill those wants? Who has interests rival to the player group? Who wants what the players can give? Bards and rogues might have a good chance to shine here.
As for exploration, of course physical barriers and climate will affect how your trade routes are shaped. Wilderness adventures are a staple of D&D, but here you would really depend on it. Players could explore the frontiers mapping new routes and finding new societies to trade with. Rangers and druids might be useful here. Of course, things like flight, teleportation, and gates can circumvent a lot of natural barriers.
Now, trade relies a lot on natural resources. This is true even in D&D, which relies a lot on gems and other expensive components for spells (prior to 4e) or ritual components (in 4e). Can you imagine how these spellcaster requirements might influence trade? You could make an adventure out of feuding societies trying to commandeer access to such precious resources. What if, for example, somebody took control of the dwarven gem mines that put out all those expensive diamonds used for resurrection spells?
So I guess the big point here is that there are already a lot of D&D/fantasy tropes you could extend to a merchant campaign. I’ve heard of people structuring event based campaigns like dungeons. Making certain choices is like opening up a door leading to a new possible event. Like, if you pursue A action and B action, then C or D results may happen (depending on the roll of the dice and other variables). If you pursue E or F actions, then G or H results occur. I’d imagine such a construction might be like a social web of possibilities.
No reason why you couldn’t extend that way of structuring your campaign to a merchant adventure path. For example, say that there is a brutal barbarian tribe that’s threatening your trade route. Do you take them on yourself? In which case, you probably got a bloody war on your hands and may lose many of your own men. Do you call in some foreign mercenary band to fend them off? In that case, you now have a new ally, but you have to make regular payments to them. Do you brave the unknown and possibly dangerous frontier to find a way around them? Do you walk a tightrope trying to make peaces with the barbarians?
Basically, I'd imagine an optimum merchant campaign like web of potential PC choices and the results of their interactions with the world. Where they go and who/what they deal with to make trade provides the adventure hooks.
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