Christopher Rice has had a few pyramid articles and blog entries that could be useful.
Pyramid 3/87 Purveyors of the priceless and the
designer notes
A DF blog post about
merchanting
DF 16 Wilderness and 17 Guilds have some useful things.
Social Engineering has a lot of useful things about reactions and influence rolls
Gurps Traveller Far Trader for 3rd has a lot of economics and trade discussions though set in a sf setting.
Spaceships 2 has rules for running a merchant ship that wouled apply to caravans.