The marketplace allows a player to choose the next Special building, so the 'extra' card is there to allow the person activating the marketplace to always have a choice.
From the rules:
After this, the player may look at the two top cards in the Special buildings pile and return them to the deck in any order. At the end of the game, the Special buildings space is never empty. This allows for the Marketplace: the visitor must be able to influence the order of the buildings right to the end of the game. The next player can start his or her turn while the player is looking at the Special buildings.