You just never know.
Think of any car, no matter how rare, that you might want to own. Almost anything you want can be found in used car listings somewhere, whether a classic Mustang or a pre-owned Lamborghini. The truth is, somebody, somewhere, probably wants to sell the car you want to buy. The trick, of course, is finding it.
Every once in a while, a car so rare comes up that it should be crossing the stage at a Barret-Jackson auction, rather than showing up in the classifieds.