When a player has a trade fleet and sells resources to a port doesn't that mean the port now has some of that resource because of that player. I played another game where when someone sells a resource to another port that port then gains that much resources meaning that another player can then buy those resources from that port. Depending on the rarity of the item at the port determines the price. Or the value of the good to that port. i.e a player sells 80wood to tazgos, therefore, tazgos receives a new shipment of wood from Aiora of 80wood. This means that another player or the same player can now buy that 80wood from tazgos because it was sold to that port and now the port has possession of it instead of it just disappearing. Now, lets say tazgos has 100k wood from the trade of multiple players, that wood now becomes worth less and less to sell at that port because they have so much of it. It also becomes cheaper to buy it there too. But at minimum, the cost can only drop to the base cost, + 1 or 2. You guys decide. This makes the game a lot more realistic in terms of trade. The resources the port has can also be used to build port building by the governor or such depending on the what the governor will be able to do in the later features.

A difficulty for players with larger fleets would be the need to constantly change trade routes as ports developed gluts of goods and prices dropped so that profits became minimal (or maybe even negative, a minimum price would be artificial). Xepshunall has proposed elsewhere a fleet management system which could reduce the effort to do this but anyway it would need much more active trading management and the means to do it efficiently.

Another problem, how would goods be consumed so that stockpiles wouldn't keep building up forever? Maybe this could be based on population or on the owners' activities under a new port ownership system. But at the moment there is no demand for goods built in.

A very interesting proposal, how to make the economy more realistic, but I suspect there may be quite a few pitfalls to be avoided.

The goods I think should be constantly dropping as influence does going down maybe 5% each day if the amount of resources is above a certain point. This along with other peoples thoughts to try to counter this would be great. Maybe the merchants (officer i.e, admiral, captain) can have more of a crucial role in the game meaning that merchants can change the route themselves if this happens. This means the player could have to set a backup trade route. So an example would be: Tazgos - Tortuga and as an optional secondary Aiora- Vaasburg

well, in fact, the port does get them. It just does not sell them back
The rarity is used to get the prices to the rest of the ports. Then at the daily update, we delete them

I do not see a point for a port to sell other resources. It will mean a totally different market system with the trade routes we have now. This system is fully automatic and has many ways that can be utilized - I have tested it extensively in my test account.

At the end of the day, with total cargo of 1 fleet being at max 700 crates, who is going to manually go around ports checking prices and manually trading ? Especially by the time that there is already a more sophisticated system in place that lets you do the exact same thing without the requirement of micromanagement.

If you wonder how can this be accomplished, I suggest you take a look in how you can utilize warehouses. You can stock your resources there till you get a good price