Trading goods is a great way to make money in Mount & Blade: Warband. In this article, you learn example price ranges and thresholds (to buy under X, to sell over Y) for the most profitable goods. You also learn an example trade route that goes around all Calradia in Warband. The tips and informations originate from Tale Worlds Forum and are just summarized here.
You can learn all this info in game, by doing trading, using the asses good deals option in the marketplace of the towns, using the Merchant's Ledger, talking to the guild master about trade and production of the different cities, so consider the information below. Also, there are a lot of trading opportunities not all can be listed here. Thus, feel free to experiment and take on the opportunities you see.
The following price ranges provide an orientation on profitable prices for buying and selling. The prices refer to "normal" goods without special quality modificators.
If you see goods under the threshold buy it. And only sell if the price is high enough. If you need to empty inventory, any price over the buying threshold is profit, so it's OK to "dump" inventory if you know that the next cities in your route don't pay well for those goods.
BUY under X means buy when the buying price is below X.
SELL over Y means sell when the selling price is over Y.
Buy these to avoid travelling empty, if the above are not available.
If you don't have much trading skill in your party, or the economy in the towns is a bit off because of wars etc, you might not get prices as high, so do drop them by ~10% or so, e.g. SELL iron over 270, SELL salt over 250 etc. For buying, avoid buying for higher than the threshold. Just move on to another city.
This trade route is originated from Me, Floris After Action Report by Monnikje. It's a complete route around Calradia, focusing on the most profitable (small) routes and most profitable goods (only buying "filler" goods in between to avoid travelling empty).
If some goods are not available or not available below threshold price, wait for a day or just move on (usually you should move on). Sell to the other merchants if the goods merchant runs out of money and the price is still high enough (the arms/armor/horse merchant).
If you have inventory space between towns (could not get enough cheap goods), stop in villages on the route (don't go off it too much thou) and buy cheap items. Also, buy variety of food from those villages (to keep) army fed and morale high). This money will help the economy of the town as well.