The trading posts, are on each port such as dragon port , the parrot port etc.
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 6th outlying island is hidden until you collect all 5 map pieces (one from each of the trading islands). It only comes into play after you have driven off Captain Crawfish and have the Treasure Location card from the Governor. Travel to Skullduggery and use the bone shovel to dig at the marked location.
Collect the 5 map pieces, one from each of the trading islands, and the hidden island will appear on the map. You cannot do anything there until you have defeated Captain Crawfish.
When you have all five map pieces from the 5 trading islands, select them from the item list and they will combine, and the new small island (Skullduggery) will appear near the Pirate Outpost. You can't do anything there until you have beaten Captain Crawfish and returned to see the governor.
There is a dock at the bottom side of the small island, Skullduggery. The island appears when you have collected all 5 of the pieces of the Treasure Map.
When you have all of the map pieces from the 5 trading islands, the 6th island (Skullduggery) appears near the Pirate Outpost. You cannot do anything there until you have beaten Captain Crawfish. This will require the largest ship, the Phoenix Warbird, as well as hiring the Shipwright and Cannoneer. Once you have beaten Captain Crawfish, go to see the governor and he will give you the directions to the buried treasure on the hidden island.
There is one piece on each of the 5 trading islands. You can collect them in any order, so any of them could be your second piece. (see the related question)
about 5 seconds
When you have all of the map pieces from the 5 trading islands, the 6th island (Skullduggery) appears near the Pirate Outpost. You cannot do anything there until you have beaten Captain Crawfish. This will require the largest ship, the Phoenix Warbird, as well as hiring the Shipwright and Cannoneer. Once you have beaten Captain Crawfish, go to see the governor and he will give you the directions to the buried treasure on the hidden island.
When you have all of the map pieces from the 5 trading islands, the 6th island (Skullduggery) appears near the Pirate Outpost. You cannot do anything there until you have beaten Captain Crawfish. This will require the largest ship, the Phoenix Warbird, as well as hiring the Shipwright and Cannoneer. Once you have beaten Captain Crawfish, go to see the governor and he will give you the directions to the buried treasure on the hidden island.
When you have all of the map pieces from the 5 trading islands, the 6th island (Skullduggery) appears near the Pirate Outpost. You cannot do anything there until you have beaten Captain Crawfish. This will require the largest ship, the Phoenix Warbird, as well as hiring the Shipwright and Cannoneer. Once you have beaten Captain Crawfish, go to see the governor and he will give you the directions to the buried treasure on the hidden island.
Once you have all 5 map pieces, the new island (Skullduggery) appears near the Pirate Outpost. But there is nothing you can do there until you acquire a Phoenix Warbird and defeat Captain Crawfish in battle. Once that is done, return to the governor in Fort Ridley and he will give you the map to the treasure on Skullduggery, and a shovel to dig it up.