Usually such folks have utterly no idea of what stamps are worth but have some outlandish idea of the value of THEIR holdings.
My guess is that these are the same kinds of people who overspend on lottery tickets, constantly thinking they're going to strike it rich.
Another guy brings a rock he found in his back yard to shows. When someone gives him the 'But, it's old' routine he picks up the rock and tells them it is probably a billion years old. What will they pay him for it?
He could tell them it's about 14 billion years old (give or take), because it is. So are you, or at least the material you're made of is, assuming it was made in the Big Bang along with everything else in the observable universe.