Trading and Combat

I'm making a "Pirates of the Caribbean" board game (obviously this one is just for fun and only to enhance my own gaming experience) that is based mostly on the second two movies (but also includes elements from the first one- I haven't seen the new one). Basically it puts the Pirates against the East India Trading Company.

The main objective of the game is to make a profit, mostly via trading. I'll come back to this in a minute.

There is also combat. There are three types of ships available- (names yet to be created)
A fast ship- (speed rating: 3. Hull Rating: 9)- 2 cannons
A mid ship- (speed rating: 2. Hull Rating: 12)- 4 cannons
A slow ship- (speed rating: 1. Hull Rating: 16)- 6 cannons

Speed rating dictates how fast a ship is. Currently 3 = +1 to dice roll; 2 = same as dice roll; 1 = -1 to dice roll.
Hull rating dictates how much cargo a ship can hold, and also how much damage it can take. Each number refers to the amount of crates it can hold. When a ship gets damaged, damage tokens are placed onto the "cargo hold" (i.e. the more damage you have, the less you can hold). If the cargo hold is full and you take damage, you need to throw crates overboard, thereby losing money).

Combat works on the basis of cannons. For every cannon a ship has, it can roll that many dice. Now one of two things can happen (not sure which one I will implement- suggestions wanted)

If it is a 5 or 6 (for example) it is a hit and if it is a lower number it is a miss.
Defender rolls dice, the same thing as 'Risk'.

When being attacked, a defender can choose to fight back or retreat. If they fight, turns are taken to roll die to attack until either one ship sinks or one flees the battle. If defender chooses to retreat straight away, after the attacker gets a free roll, they roll a dice and move away. The attacker may choose to follow the ship, in which case they try and roll the same or higher, or they can let the other player retreat.

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I need help coming up with a trading mechanism for the game. I know it is the main part of the game, but I don't want it very complicated...

Pretty much, you can trade gunpowder, rum and spice (I'll probably end up having one or two more things) between ports. I want an element of risk when buying things (i.e. a player may not necessarily get the same/better price for something that they paid to buy it), if that makes sense...

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Each player starts with $15 000 and 3 ships (one of each class)
Extra ships cost $20 000 (when buying a new ship, a player may choose it's class) and cannons cost $1000 each (you don't start with any).

Obviously you make money by trading.

Another aspect of the game is cards, used mostly for combat advantages (much like those themed Risk games), but also to help or hinder trading.

Pirates cannot trade at Port Royal and East India Trading Company cannot trade at Isle de Meurta without first conquering the ports. Each team has advantages (eg. EITC can collect port taxes, and pirates can plunder ports, etc.)

For example, Pirates get rum for half price on the island where Jack is stranded (I have the name, but I can't remember it right now) and the East India Trading Company get cheap spice on Pelegosto. Each team has a card somewhere in their deck to nullify this for the other team. Other cards give you roll advantages and what not.

The last aspect of the game I want to talk about is Davy Jones. His Chest is hidden somewhere on the gameboard, and the player who has it gains control of the Flying Dutchman. (Fast ship with 4 cannons, but cannot carry cargo). The players also have special "green" cards in their card deck that can only be used when The Flying Dutchman is under your control (eg. Using the Kraken). However, these cards are not very common.

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Sorry, that is a lot of info to take in, but I'd like you to fully understand the game, and hopefully all of that makes sense.

The main thing I'm looking for is a good trading mechanism that would work well in this context. Any other ideas or opinions on other aspects of the game would also be greatly appreciated!!!

Thanks a lot!
-Matt