Quite simply, when enough people are nostalgically drawn toward a certain thing—and have money—it becomes valuable. Dealers call it the “20-year rule.” Typically, about two decades after a particular toy is popular, the kids who played with it have become adults with a yearning to recapture their youth. “They want their childhood back and one way to do that is to buy a piece of it,” said James Gallo, toy collector and owner of Toy & Comic Heaven, in Willow Grove, Penn. “You have some...