The key in this book was used to open the door in the drawing room to go to the Other World and was also the bait to make the Other Mother's hand fall down the well.
Buttons are in Coraline, some on characters, like the Other Mother, and two that were supposed to be sowed in Coraline's eyes but didn't happen because she escaped.
Marbles were shown in the book as the shapes of souls and were one of the requirements to win the exploring game and win Coraline's freedom from the Other World.