At four of the islands, there is one cargo that you can buy cheaply. By following the clockwise circular trading route, you can make more profit.
Cheap Cargos :
Of course, you can sell salvage items found in the water at any port, and it is all profit. The "clockwise route" means that you buy at the lowest price and sell at the highest.
Parrot Port has the highest price on medicine. It is the island in the easternmost island.
The price is highest at Bouffant Bay, 2 to 3 times what it costs at Dragon Cove.
At Bouffant Bay, with a high price around 40. You can buy it at Dragon Cove for as little as 13 to 15.
Sell your salvage for doubloons, or buy cargo to transport to a different trading island. For example, buying silk at Dragon Cove, then sailing to Bouffant Bay and selling it for twice the price.
You can sell whatever you want that you may have salvaged or bought. The highest price paid at the pirate outpost is for Grain (which you can buy cheaply at Golden Harbor). At the Pirate Outpost, you can always buy Spice at the cheapest price.
From things you sell at a higher price from what you paid for.
You can get doubloons by trading, but the price changes very little at one island. The most profit comes from buying cheaply and selling for more somewhere else.
The price changes throughout the game, but is practically always the highest at Parrot Port. But it can sell for as little as 32, or as much as 40. The highest price for medicine at Golden Harbor or the Pirate Outpost is between 25 and 35.
Start from any island. Buy things at the lowest price like when they are 14 doubloons. Sell things at the highest price like when they are 32 doubloons.
In a nutshell, they bought things at a low price from one place, then sold it at a higher price at another place.
Stone Island jackets can be purchased at a variety of locations. The best place to start is online at perhaps Amazon. The Stone Island website also has them for sale for a higher price.
The price of land depends on which island, and WHERE on the island.