I plan on detailing two or three specific merchants who will conduct their business across the primary region the PCs will be operating in. I'm a big fan of trying (not always successfully) to turn questions of "how" into tools for use in the campaign. Each one will be different, have a different "method" behind his success, and will serve various other purposes. And, of course, the quirks and methods of each are an extra layer the PCs can peel back down the road. I like dropping little revelations in odd places and letting the players realize what and who they pertain to and what the ramifications are.
The first is the retired adventurer who is pretty powerful in his own right who has connections with the ruler of the region so can serve as Eyes for the crown and as a good source of political knowledge to the PCs--he's going to rely on himself and several highly trained guards. He's going to have solid information on various areas, will be generally honest, reliable, and has the authority to issue and collect Warrants for the crown (bounties). Of course, anything he knows the ruler will soon know as well. He gets fresh pigeons at most of his stops.
The second is probably going to be half-lunatic wizard (who I have plans for later, I think, just to tie in an idea down the road to the PCs' beginnings) who also sometimes trades with many of the monster tribes of the region since he doesn't view them as being any less a potential customer than anyone else (and likes seeding them with items for destabilization reasons). He relies on his contacts and goodwill the various monster tribes for passage and falls back on escape magic. His most noticeable trait will be that he always, always lies when it comes to his schedule, destination, and route. He happens to be searching for a very particular item ...
The last will be an underworld type who travels as a merchant to help mask the fact that he's also collecting "fees" from small timers for a larger organization (debts, loansharking, gambling) while also delivering contraband, drugs, outlawed poisons, slaves, and stolen objects. He will serve as a means into the criminal element of the region should the need arise. His means of getting from place to place is a large number of guards, not so highly skilled, who use poisoned blades, explosives, and trails which (through bribes) have not been marked on any common map. He can, and will, buy most anything and will be greasy-friendly but capable of getting his hands on most anything given time.
Eh, just rambling because it's late. I just like the idea of the PCs running into basically the same merchants over time and seeing how they take to each one and where the possibilities go.
