The marketplace is the building in the Abyssal Palace Outpost that allows you to trade items for other items. You have a number of free trades each day, depending on your marketplace level. After you have used all your free trades, you either have to wait until the next day before you make new trades, or you can use one Coral Dubloon for each new trade after you have used all your free trades.
The trades refresh every monday. Some trades stay for more weeks, and some dissapear.
This is the stats of the marketplace:
|Level||Coral Dubloons needed to upgrade||Number of free trades per day|
You do not gain power by upgrading the marketplace.