In my latest game I have 3 merchants: Collecting in Seville and two transferring in Genoa and Tunis.
In Genoa I am moving 1.2 ducats towards Sevilla.
In Tunis 0.39.
At the start you should always have a merchant collecting from your "home" node. That is the node that has your capital in. Your second merchant should be sent to either the node with provinces you own or the next richest node "downstream". In this case it's Genoa.
My third merchant (after I unlocked the Expansion idea) and sent him to Tunis. However, Alexandra is actually worth almost twice as much (you can't see it in the screenshot) at 4.6ish. But you don't want to send him there.
Here's why: Let's say you merchant draws 1 ducat from Alexandra to Genoa; its connecting node. You will not see an increase of your trade from 1.2 to 2.2 because that 1 ducat will be divided up amongst all other nations that are collecting from that node (all the flags below the node). Since you have no provinces in the Genoa node your trade power (how much you can extract) is minimal.
Thus, while Tunis is not as rich, I am more powerful since only weak North African nations are collecting from it and only Aragon and I are withdrawing. Aragon pulls 0.76 towards their home node, I pull 0.39 to myself.
Light ships on patrol create Trade Power. In the screenshot both Genoa and Tunis have higher trade power due to large patrol fleets with high maintenance. 6 Ships for Genoa (southwest of Provence) and 3 for Tunis (South of Sicily).
Against weaker powers (such as the Central African nations in the Ivory Coast node) you can actually dominate the trade node with just a few light ships. You can't see it but with just two ships on patrol I was moving about 70% of the trade towards Sevilla.
There is a maximum amount you can move away from a node and that is based on you province trade power.