They change from day to day. However, there is always one place where the cargo is cheapest, and another where it is always the most expensive. These locations are clockwise from each other.
Buy grain cheaply at Golden Harbor, sell at Pirate Outpost
Buy spice cheaply at Pirate Outpost, sell at dragon Cove
Buy silk cheaply at Dragon Cove, sell at Bouffant Bay
Buy medicine cheaply at Bouffant Bay, sell at Parrot Port or Golden Harbor
You can actually sell cargo or salvage anywhere, but you will make more selling as shown. And you would never buy a cargo at the place where it always costs the most.
Skulduggery Island
hyu
Skulduggery
You beat the skulduggery Island. At the end you get it.
You Stand On It ...
on dragon cove
in the tradding post (sorry if this is wrong it might be a diffrent island)
You can buy new boats on the poptropica "China" island, Dragon Cove. It's to the North-West of the main island.
You have to upgrade ships. The more expensive the ship, the more cannons it will have.
You should sell silk on Golden Port
your asking to cheat i wouldnt ask if i were you
Stand on the sinking dragon statue in Skulduggery Island. Then jump onto the tail to get the mallet.