For a trading strategy it is good to have a half-dozen or so extra mounts to keep your speed up, as well as traveling with a mid-size (<100) party of all mounted troops. You will want to accrue plenty of Inventory Management to have a goodly number of slots available for merchandise. Here are a few favorites of the top of my head and from memory:
I find that Tihr pays well for many lucrative commodities: Oil, Iron, Furs, Velvet, Spice, Wine, Ale. Dried Fish can be had there for a song, and of course they are always buying slaves. If you get too much Spice or Velvet Sargoth will pay decently for them also.
Get Furs and Iron in Vaegir towns and villages, plus Tools and Wool when they are cheap. Vaegir towns pay well for Oil and often Wine. Beef is a bargain between Khudan and the more coastal locations, which often also have decent Dried Fish prices.
Oil is often cheap in the most inland Rhodok towns and villages and they will pay well for Furs, Iron and Wool. Cheese is usually a good deal in Yalen and can be sold at a decent profit in the more distant Kerghit locations. Velvet can sometimes be had at bargain prices in Jelkala. Plus wine there and around Veluca. Narra will pay well for Beef, Fish and Tools. Get Spice and Salt when they are cheap in Tulga and nearby Kerghit towns and villages.
A couple of spins around the map buying and selling, plus collecting taxes from fiefs and towns that I own is always profitable enough to take time out to siege 2 or 3 towns and well-defended castles with 250-350 heavies and missile-men without going in the hole. Checking all villages and towns for prices of goods will help you get a feel for what good prices are for the various items -- these will also improve with your party trading skill and relationship with the location.