The vast majority of the *****ing in this thread has to do with two trade goods, velvet and spice. They also cause problems in WB. Too profitable.
Then there's the issue that bulk commodities can't be moved in sufficient quantities to make them profitable.
Finally, caravans are boring to escort.
Here's how I'd solve these problems.
1. Add more velvet and spice production facilities to cities (more common, lower prices), raise the baseline acquisition cost and lower the max profit (bringing margins in line with other goods).
2. Add a 10% discount to caravan purchase price for all low level bulk commodities and food items (grain, flour, etc). Raise baseline price of low end and food items in all cities by 25% (raising margin on caravan sales, nukes it on human trade).
3. Scale carrying capacity of caravans given given lowest potential caravan buy price and highest potential sell price such that the expected net profit of caravan in optimal conditions is roughly equivalent across all goods (meaning caravans might carry 1k units of grain and 10 units of velvet). This optimal condition expected net profit should be set at a number in the range of 5k to 40k. Given implementation of suggestion 4, I would pin optimal condition net profit at 20k or less
3.a this adds skill to the caravan game, as it requires intelligent selection of origin and destination based on local factors for a wide range of goods rather than semi random destination selection for a single class of goods where skill lies in searching out optimal origination points based on distance.
3.b all caravans with a destination of a different nationality from their origin will be subject to a 10% not waived by trade permit.
4. Player can send caravans without escorting them to destination. Caravan will use standard caravan group type and can be ambushed by bandits or enemy parties as per standard implementation.
4.a player can send X number of simultaneous caravans, where x = the highest applicable trade skill in party. A party member with 10 in trade can send 10 simultaneous caravans.
4.b caravans can be sent with the order to trade once or to loop between initial origin and destination with a separate good type on each leg. Looped caravans are subject to a 20% tax on net profit which is not invalidated by trade permit. players can only loop caravans once they have reached trade 5 (prevents newbies from having cash suicide by giving them time to find profitable routes)
4.c loop caravan weekly profit is calculated on wage day and all profits and loss are displayed to player that day. Profit or loss is disbursed to player on that date, with debts incurred charging a rate of 35%. one time caravans disburse monies on day of arrival.
4.d caravan routes are displayed on a single interface screen in notes menu with all relevant info (making it visible at mayor screen by clicking on mayor face). Looped caravans can be adjusted from any mayor, with changes taking effect at end of next trip.
Taken as a whole this will result in a more robust and interesting economic subgame less subject to exploitation and requiring less micromanagement from the player. Profitable trade will depend on knowledge of relevant local conditions rather than market seeding. Profit will be lower and more stable. All in all I think this is a generally good solution that makes expensive **** affordable without being grindy.