Johnny, I second what David said. Heck, just offer $1,000 sight unseen and see if that lights a fire. If that sounds too risky I'll put up the cash and we can split the collection 50/50
As far as the original topic, I would agree that there are many unknown collections. For my specialty, 19th century non-sports, the guides often mention cards that exist but none have been sold in the last dozen years or so. One example is the N9 Allen & Ginter Flags of Nations card of Roumania. The book American Tobacco Cards mentions 6 known but the only public example is the one I have that's not one of the 6. I just think they'll eventually end up getting sold when the collector passes on.