Great card.
While everything is for sale at the right price, I could cite quite a few from my collection that would not be sold absent an insane offer. Some of them:
1904 Jeffries-Munroe cabinet card by Sporting Life magazine:
E80 Jack Johnson that belonged to Jefferson Burdick:
Benny Leonard "Flying Fists" die cut. One of two known specimens and the other one has the tab removed:
1938 American Sweets Joe Louis:
1951 Ringside salesman sample with Marciano RC:
This one isn't as impressive, but it is rare and I just happen to like it:
