Microeconomics and edition wars aside, the important question is this:
What does the player expect to get out of having an exotic weapons merchant?
If it's just background info and he doesn't expect anything more, then you can file it under the "duly noted" category and get back to the game. Occasionally he finds a cool weapon or gets to make a sale.
If he wants to craft his own unique weapons, then you'll have to come up with some sort of crafting rules (probably based on 3.5 or a variant) or tell him he's SOL.
If he wants to make extra money, either do as suggested above and count it as part of the party's wealth advancement, or give him a attribute roll (say, Wis or Chr) to make a few gold per week, as the 3.5 Profession rules.
If he wants to use this as motivation for a treasure hunter, Indiana Jones type character, then give him lots of exotic locations with legendary (but level-approriate) weapons full of traps, undead, hostile natives, and the like.
That's about every variation I can think of. No matter what the case, the easiest thing to do is to ask him before the game what he has in mind.