Although each character has a suggested course to take, and must follow it at least somewhat, nothing is going to stop you from pursuing your own path to fame. You can easily make Catalina the pirate into a merchant, or buy the greatest armor and sword for Ali and have him take down pirates, or even other merchants if you'd prefer. Just don't sink a ship from your home country, or they might not let you into their ports anymore. If you decide to make your living peacefully by trading, then you have a number of things to deal with. Naturally, you must buy low and sell high, but things are not always that simple.
Prices reflect the activity on trade routes.
Buying too much from one port raises the prices, whereas selling a lot lowers prices. After unloading a few hundred lots of Istanbul carpets in Athens, they won't want to pay nearly as much for them. Conversely, if you buy lots of artwork in Athens, they will start to charge more. These prices are not always absolute, however. A smart captain, or a captain with a first mate who's skilled at negotiation will be able to barter with merchants to get substantially lower prices.