European traders are the NPC suppliers that start acting when prices for the individual resource reach certain level. 
They stockpile the resources bringing them from Europe. They operate using contracts. Once contract buy prices reach certain levels they start to fulfill the contracts delivering you the goods you need instantly. 
 
They only do it in the ports that provide that resource naturally. For example if the port is producing hemp - they will bring you hemp if you price it right. They also can provide you with trading opportunities with towns that have high demand for hemp and does not have hemp production in those towns.
 
For example. We have heard that hemp prices in certain capitals sometimes reach 500-1000 gold per load. 
You can buy hemp from european traders at 360 - and bring it to that capital making 140-640 gold per load destroying the shortage and making a lot of money as a result.