Here are some merchant tips:
Fine tune your character to have Inventory management, Trade and persuasion. Pathfinding can be crucial to not be caught by vikings. Use
this. Being a trader allow you the ability to read without learning from a monastery, saving you 4000 gold.
Personally, I started out in Norway. I spent a great deal as a vassal of Norway and got 1000 gold per week for nothing.
A general rule for trading is to buy almost any raw materials in villages, that are cheaper than normal, and sell it at your town. Even buying wheat for 2 gold, and selling it in town for 30 gold adds up in the end. Visa versa, buy finished products like wool cloth and sell them once a village gets enough coin from trading with you.
Don't sail during winter. Look at the mountains. If it's snowy, then it's winter and you should try to spend your time trading between village and town. I spend about 700 gold in ship repair from Norway to Scotland alone.
As a trader start with the smallest ship possible and embark on
The North Sea Trade Route. The main goal is to buy as much wool from the villages in England for 30 gold, and sell it Friese for 900 gold. Norwegian villages and towns have valuable fur, tar and timber, which is needed for building large ships and your first refuge.
Your first cycle will be focused on generating a starting capital. At your second cycle around the route you will have about 10 000 or 20 000 profit.
Mind you, the Story mode will give you resources to allow you to start trading earlier than in Sandbox mode, but the down side is that certain trade hubs will not be open to you. It's recommended to start your trading empire in England in sandbox mode, trading raw resources from villages to towns, and selling manufactured products back to villages.
Once you have about 14 000 gold, you want to invest that money into about a handful of enterprises. These self generating enterprises will become the back bone and your funding for your new faction, so make sure to keep it some distance away from where you think to start up. If the faction that houses your enterprises declares war on you, then the enterprise will be confiscated and any income devoid.
Good luck establishing your new trading empire.