It would work like this.
Cyro trades stone to me.
I trades stone to Eranor.
Eranor trades art to me.
Irodenia trades metal(Iron) to me.
I trade metal to Cyro.
Sengrin trades spices and fruit to me.
I trade spices and fruit to Irod.
I trade Irodenia to sengrin.
This probably won't work but what do you guys think?
[img=http://i.imgur.com/lwXYprK.jpg?2]