My idea was simple: to craft an item from bought ingredients and then sell it for higher price than it cost me to buy ingredients and craft the item. 

I found three such items that can be crafted with positive profit:

1.Flash grenades.

2.Hats.

3.Simple coats.

Hats are more profitable than simple coats and made from same raw materials, so you can just ignore simple coats and craft hats instead.

In order for hats to be profitable we need cheap fabric. And we will be able to get endless supply of cheap fabric if we will buy the Bitterroot farm. After that the farmer will sell us 1 unit of fabric for 175 gold.

To produce flash grenades we will need beer. It's sold in the Aldlyn Bar (among other places).

We can buy the grenades recipe from the blacksmith of the elvish village (AKA Ebron).

P.S. Also, it seems like I found the best trading route. I assumed uniform distibution when I calculated expected value of profit. I also prefered absolute size of profit over relative size (i.e. it's better to have huge sum of money as profit, even if rate of profit is small). Turned out that this city in Dorgania is the best route (I ignored the city where you get items in return instead of money. Yes, it's possible to sell them, but in order to be more or less sure how good this trade route is I would need to gather data about prices of bought goods. It would require me to use said trade route many times in order to collect enough statistics and I felt that it would take too much time). I ignored time needed for caravan to arrive because I would be too impatient to wait for arrival of the caravan even on shortest trade routes.