We will not bother with any police over the system; whoever can manipulate it, let it be. We mostly want to build something that will be simulating real world market.
I like your approach Shaydo. Here is what we are trying to achieve (our objectives):
Primary Objective:
We want large profit to be dependant on ships.
Which means that a producer alone, should be allowed small profit. It will be the ships that will sell the goods at another port that will make a difference.
This leads us on a troubleshoot:
Why a player should use his ships to sell at another port, instead of waiting for the perfect time on his own(producing) port?
This is obviously something we do not want.
So, the whole point is to create the feature in a way that gold bars will be loaded in ships. We want every player to have a couple of "Treasure Fleets". If players start creating such fleets for the sole purpose of transfering gold bars, we will make the game much more interesting.
For example, Pirates will now look for these fleets. We could even add specific ship abilities that instead of the gold coin loot, they will target for the fleet cargo. Now, for a fleet that carries tools (the most expensive commodity), the maximum profit out of the cargo can be 15k. For a fleet that carries gold bars, the maximum could be beyond 1.5million.
So again, we need a way to make gold bars profitable, like the resources are. The whole problem till now, is that ALL ports buy gold bars.
On a final note on this post, we could force gold bars travel by setting specific ports as gold sellers and other as gold buyers. Or, we could return to the initial idea that Hideout has its own port. So a player will be force to sail his goods to a port warehouse anyway. Perhaps the later can be the solution. But still,we need a good market solution.