Sea Trading allows you to exchange goods with other players. This can be used to your advantage to help fulfill orders for Liang, feed Glurb, or even fulfill the requirements of some (but does not work for all) Quests. Purchasing completed items (as opposed to item components) usually does not work with time-limited Quests. When purchasing goods, you can choose to view everything on the open market that is being currently sold, or view only what your friends are selling, or view only what your alliance members are selling. Unlike Myra's Caravans or Liang's orders, you can set the price of goods that you sell. You can sell up to three types of goods to be seen by all buyers who are Sea Trading.
If you are in an alliance, you can sell one type of good to be purchasable only by members of your Alliance, or purchase goods that members of your alliance have made available for alliance members. This allows you to sell a fourth type of good, in addition to your other three types of goods sold on the open market.
Unsold goods remain on your ship until they are sold, but will automatically be sold on the second or third sailing.