What does this character do?
At his core, the merchant is a provider of goods and/or services that aren't in ready supply in a given location. Classically, this means that he's loaded his bags with all sorts of things that he's selling off to whoever has money in the hopes of having a fat purse, and that's certainly an element to play with. But there are a lot of different things that you can sell and a lot of different forms of payment that don't necessarily glitter under sunlight.
For example, consider a wandering paladin. (Yes, in the archetype sense of the term.) The character roams from town to town, always stopping where people are in distress or in danger, asking nothing more than a place to sleep and meals to eat until he's solved the problem. He doesn't want money; he wants to save lives, and achieving that goal is payment enough. But he's still a merchant of a sort, just one more concerned with providing freelance life-saving solutions. This is what he does. Or perhaps he offers his services for a modest fee and serves as a trainer, helping small settlements prepare to repel raiders and the like with what they have on hand.
The important thing that makes the character a merchant is that he's trying to provide something for a price that he sets. That price may be something that the buyer considers intangible or irrelevant, but it's still a price. To use the wandering paladin from above, he'll gladly step in to save a town menaced by bandits, but he won't help a town wage war on a helpless neighboring city. There's not a fair price on the table.
What does this profession provide for roleplaying?
Merchants always have a reason to move on. Most merchants need to move to new places, either for new stock or for new clients. They also need to keep acquiring whatever it is they receive in payment, whether it's money, artifacts, memories, names, or whatever. Even the few who have a stable business and have supply issues handled by others still want to expand because that's how you keep going. This is a good thing for any roleplaying group, as it provides a lot of good reasons for people to keep changing dynamics rather than staying static.
A good merchant often has access to some things that are either in short supply or are at least difficult to find at a glance. This isn't always the case -- the aforementioned wandering paladin might not be a great swordsman, but he's cheap -- but you can expect merchants to have something worth selling. This obviously extends to mercenary characters as well, as poor warriors tend to have very short careers as mercenaries. And a merchant will know where to get more of the relevant items, whether that means knowing the location of rare herbs of having a lot of friends willing to kill things for money as required.
What sort of characters work best in this role?
There are really two sorts of people you can find serving as a merchant, and the first is a very traditional merchant. He likes to make money, and that's true whether he's only out to get paid, considers it a duty, or just doesn't have any alternatives. This is a character who wants coin, and while his reasons for wanting it can vary, at the end of the day he does still have to get paid.
But then there's the other sort who wants a more esoteric payment. These characters want their object of affection just as much as the merchant who needs to get paid, but they have to search for clients who can pay. These are people who have a story behind what they're looking for in the first place. They also tend to be a bit high-concept and high-maintenance, though, working better as occasional guest stars rather than regular individuals.
What should I keep in mind?
There are a lot of different flavors of merchant -- the sleazy con artist, the genial trader, the enthusiastic peddler. As a result, you have a lot of range with the character. More than anything, you have a chance to really illustrate the difference between a profession and a person, as you could easily have a paladin in game terms who works as a merchant to make ends meet.
Also keep in mind that while you want something, you do not want to be the guy perpetually looking for money, secrets, or whatever other form of payment you insist upon. Occasional mentions are flavorful and establish an important character trait. Constantly harping upon the lone point makes you tedious.
Feel free to offer your feedback down below or mail it to firstname.lastname@example.org. Next week, I'm discussing liking the characters you play. The week after that, let's talk about a topic near and dear to my heart and ears: roleplaying music.
Every Friday, Eliot Lefebvre fills a column up with excellent advice on investing money, writing award-winning novels, and being elected to public office. Then he removes all of that, and you're left with Storyboard, which focuses on roleplaying in MMOs. It won't help you get elected, but it will help you pretend you did.