Mount&Blade/Trade

Red arrows indicate the destination of each type of ../Goods/. Yellow labels beside the town names describe what they produce, the buy/sell prices, profit in gold, and profit in percent.

Basics
Horses can't be traded, but are invaluable to a merchant. The more horses you carry the faster your world map speed is (up to a certain limit, naturally).

On the world map, all horses are equal. It doesn't matter if they're lame or spirited, saddle or hunter, all increase movement speed by the same amount. If you're buying horses solely to carry your gear just go with the cheapest ones, and don't just slaughter the lame ones for their meat.

Also, horses are shared within the party; it doesn't matter which hero is carrying the horses, they'll speed the whole party up regardless. This means you can use your own inventory for goods while leaving all the horses with Marnid or Borcha.

You can freely swap inventory items with other heroes in your party by talking to them on the world map. They trade at face value, so you can sell them items and get them back later without the price being inflated; note however that while you can take an item from them for 0 gold, if you give them something to hold for you they'll demand full price for it if you ever want it back.

Merchant Walkthrough
Giving your character 1 or more in Inventory Management is a must. This gives you a good amount of storage space. Having a merchant background is probably the easiest for being an effective merchant.

The first major barrier is your small wallet. If you sell everything you have (including armor and weapons) except your horse you'll have a good bit of cash to work with. Alternately, you could take part in a tournament to get the money you need. Either way, buy the cheapest horse available as soon as you can.

During the early stages of building your mercantile empire you want to focus on the cheapest products with the highest profit margin: Salt, Wheat, and Smoked Fish are perfect for this.

Here is one example of a simple and efficient trade route that wastes no time backtracking between destinations: Since you're not in any state for combat you should run from any enemies. Be especially careful of mounted enemies, as they are sometimes fast enough to catch you.
 * 1) Wercheg: Buy Fish
 * 2) Uxkhal: Sell Fish, Buy Grain
 * 3) Tulga: Sell Grain
 * 4) Return to Wercheg and repeat

Visit the armourer of every town you pass through or buy looking for Padded Cloth (any quality); it spawns randomly for each new game, but once you've found somewhere to get it from that place will consistently sell it. Keep track of what place that is when you find it, as you'll be needing this later.

Any time you have about 200 gold spare buy extra horses, the cheapest you can find. Try to have at least 1 horse per 5 goods. This will serve to increase your speed, meaning you can venture further afield to carry more exotic goods more effectively while also being more able to escape enemies.

Once you have two or more horses and about 200 gold you should start touring the taverns of Calradia in search of the companions Marnid and Borcha. Borcha's abilities are very useful for moving more efficiently on the world map, while Marnid's Trade skill will make trading that bit more effective (the other two heroes can also be helpful, but they do not come with any skills that will directly aid your trading career). Give them a horse each (which optimises your map speed) and set off on your adventures. Bear in mind that their skills will stop working if their health falls to 30% or lower, so you may wish to keep them out of combat as much as possible.

If you keep this up you'll become progressively richer, eventually to the point where you can buy out a merchant's entire stock (as long as you have enough points in Inventory Management). Now you can venture further afield more effectively.

You could then expand your trade route like this: Keep buying horses with your excess cash. They will optimise your map speed, allowing you to add in the missing towns to your route. Increasing your Pathfinding skill will boost your map speed even further.
 * 1) Praven: Buy Ale
 * 2) Rivacheg: Sell Oil, Buy Meat
 * 3) Khudan: Sell Ale, buy Furs
 * 4) Curaw: Sell Meat, buy Iron
 * 5) Suno: Sell Furs, buy Oil
 * 6) Sargoth: Sell Iron
 * 7) Return to Praven