Way back in the late '70s I can recall board games centered around trade of this sort. Anyhow, I think its a good cover for adventurers, though depending on the milieu, it could be very dangerous if they start chiselling into say a lucrative business and the other merchants start to catch wind of them--the contemporary example would be the drug trade, though in the middle ages and before the spice and silk trade was rife with assasination, armed robbery if not all out war between merchants.
Back to your question--sounds like others have answered yr basic questions--just keep in mind if the PCs start bringing a heretofore rare product into a specific market, they possibly run the danger of glutting the market, thus lowering their profits. Not sure too many DMs take that into account. The party I DM took a load of tobacco to some lands to the West where it is rare, doubling their money, getting in good with local officials. Good cover for their spy mission. They never did it again--yet--and thus, prices there are still high as is demand.