Ran into the same problem recently, except I'm the player. The Mystara setting has some trade rules somewhere - I'd guess look at info about Darokin. Played around with those rules a bit ... they don't work that well for regular trade activity in my opinion.
In particular here's the sort of level of detail I want:
(a) buying prices in locations
(b) selling prices in locations
(c) costs of trading (hiring guards, mules, wagons), lodging in town, food for animals and guards
(d) duration between locations and time in town to trade
(e) market modifiers as you satisfy supply/increase demand, etc.
The Mystara system has (a), (b) and (d). (c) you can get from calculations using the 3.5E PHB prices. However, you then realise that the duration can be too small and the end result is profit per year can be too high.
I think my DM will end up fudging some numbers based on the Mystara rules.
Anyway, if anyone knows of trading systems with this sort of detail where profit doesn't end up being exponential long term, that would be great. (Old One's system is not too bad, but there's no direct account for how long it takes to get between (a) and (b).)