Note that even if the Buck Rogers character and the original material in which he appeared (newspaper strips, right?) are still protected IP, that doesn't necessarily mean that licensed Buck comics are owned by the same party. The licensed comics (assuming they're not just strip reprints), as a derivative work, are separate creative works, and can be owned and sold separately to the Buck character or original strips -- and might in fact have entered the PD on their own even if the character and other works featuring him have not. (Cf. the classic example -- the Fleischer/Famous Superman animated shorts are in the PD because their copyright was never renewed when it was required, even though Superman and most of the original Golden Age Superman stories are still owned lock, stock by DC Comics.)
I express no opinion as to whether the Buck comics actually are in the PD. I merely note that the analysis doesn't end simply because Buck himself, or earlier work featuring him, is still protected.
--Cliffy
P.S. First post. Hi, everybody -- I love the site!