From the Super Mario Wiki
The Boo Bell is an item that appears in Mario Party 2 (and goes on to appear in Mario Party 3). It rarely appears and can only be obtained in an Item Minigame. Its use is to ring for Boo and ask him to steal coins or a star, depending on how much the player can afford. In Mario Party 3, it appears more frequently, and can be purchased from Koopa Kid.