You trade going clockwise. This allows you, most of the time, to buy something cheaply and sell it for much more at another island -- the one clockwise from it.
Dragon Cove : always cheapest for silk, pays the most for spice
Bouffant Bay : always cheapest for medicine, pays the most for silk
Parrot Port : nothing cheap to buy, pays the most for medicine
Golden Harbor : always cheapest for grain, pays second-most for medicine
Pirate Outpost : always cheapest for spice, pays the most for grain
You keep trading to get enough money.
keep trading
you have to get doubloons by trading grain silk medicine and spice (go around all of the islands on skullduggery)
You go on the island you want to sell on and go to the trading post
To trade and sell on island on skullduggery island you first must be on one of the 5 islands and walk around you should see a post that says " TRADING POST"
The trading posts, are on each port such as dragon port , the parrot port etc.
On top of the barrels inbetween the first house and the trading post.
Collect the five map pieces (one on each of the trading islands) and join them to see the sixth island, Skullduggery. Once it appears on the map, you can actually go there, but nothing happens there until the end of the game. (see the related question)
you need a lot of dabaloons. You can get them by finding salavage and trading goods
Either Parrot Port or Golden Harbor, usually Parrot Port. Prices constantly change. Parrot Port is the exception to the clockwise trading route because nothing is ever very cheap to buy there.
The shipbuilder (Ship Shape Shipbuilders) is just to the right of the Trading Post.
Go to the hanging ferns. Jump on the 2nd, 3rd, and 1st ferns. Also, do your trading.