Now in 1.4.2 you have to avoid buying too large quantities because of hidden tariffs.
If you trade only one type of goods, like grain, you buy it for 9 denars and when you arrive at the town you want to sell it, it says you can sell it for 17, it is valid only for the first few batches.
If you sell 1000 units, maybe the first 50 will be sold at that price of 17 but the price drops quickly as you sell more units because the game does not treat it like you sold 1000 pieces in one go for the price of 17 a piece. No, the game treats it like you sold every single piece of these 1000 separately and for each piece sold the price continually drops as they get more of that type of goods to a point where it is cheaper than what you bought it for..
So basically the best way is to buy goods at their cheapest market value like grain for 9 or fish for 9-10 and dont invest more than 1-2k on it so you limit the loss that you incur as you sell too much of it in that city.
Otherwise if you buy in bulk for cheap, you need to sell a bit here and there small quantities in different cities.