Values come in two flavors, private and common. In a world of private values, your value for the item doesn’t depend at all on what others think it is worth. Thus your value of a personalized signed copy of The Art of Strategy doesn’t depend on what your neighbor might think it is worth. In a common value situation, the bidders understand that the item has the same value for all of them, although each might have a different view as to what that common value is.
