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"
Grains, spices ,medicine and silk. Your Welcome
Simple. There are trading posts at each island all you have to do is sell and buy things from the post.
You go on the island you want to sell on and go to the trading post
You have to sell spices silk and other stuff
You can trade your ship in when you buy a larger one.
you go around to each place and sell your cargo
buy all the grain at parrot port and sell it all at pirate outpost and so on
sell and trade with other islands (on one island what ever is cheaper on one island is worth more than the next ).
there is no easy way to make money you can trade and buy stuff cheap and trade and sell it high of just find money while your sailing.
you sell or you can buy grain, spice, medicine or silk
anywhere on the map
What I do is start at Bouffant Bay then I buy all the Medicine I can hold. Then I go to Parrot Port and sell it all for a great profit. I beat the Skullduggery Island but when I'm bored I go on poptropica and keep trading those things and so far I have 2,641,998 doubloons. =D