Ah! I see, I read your text but clearly didn’t grasp that! Yeah, that would be a bit of a problem. Could go fishing I guess 
My ideas were along the lines of each city generating a variety of resources, and consuming an amount of those resources and the difference being effectively trade-able/usable. But then circumstances/events during the game may impact on the generation of those resources. For example, a particularly hot/dry summer effecting crop growth, or flooding, infestation of insects or whatever. Some of the items produced may of course be needed to create other resources which are perhaps perceived as luxury items within the cities, but equally in demand for the success/growth of those cities and equally are trade-able. Higher population in cities may influence specific demands, some basic others luxury, perhaps the higher the population the more luxury items are required. Cities may want some base/raw materials at some point in the game to enable them to start manufacture of other goods/luxury items themselves, this would subsequently effect the value of goods of that type being brought to the city (PING modal appears on the screen and annoucnes City XYZ has just started manufacturing Good 123… you had a hold full of this and were heading there… GAH!!!)
I also wondered about reputation/standing with the cities as a trader. For example, if you say you will be there within x days with a specific quantity, but then take longer, some goods perish, and you don’t deliver, maybe you don’t only lose money but also it affects your reputation. Getting contracts for trade could become harder with a lower reputation, or maybe you get contracts which are not worth as much because they don’t feel you can be trusted. Equally, delivering goods which enables city growth may increase your standing and you may be offered more valuable contracts, but with tighter deadlines, or needing quantities that you can’t take all in one go without upgrades to the ship…
The more I thought about it the deeper the hole I dug and I realised I had moved slightly away from sea-trading to city building! I also reminded myself that this was your game and not mind so I told myself to shut up and I stopped at that point. 
I will happily share anything I find regarding the courses, most of them seem to have a supply/demand section as it happens, how much it would directly relate to your game I’m not sure… seems to be a large topic in general and as I said, certainly one I don’t know really anything about.
Good times.