I think I have part of it figured out:
The harry potter books are things that can be read by kids over and over again. Why get rid of them if they can be used again???
Tom clancy, whether you like him or not, is kind of disposable. Read it once, you get it, and you move on. Better to sell it than to let it sit around.
Bill and hillary, on the other hand, are books that were bought out of 'obligation' to the cause but never actually read. Sure, maybe they read a hundred pages or so, but even the die hards couldn't digest it all. So they put them down and told themselves "one day I will finish that book." So there's no reason to sell it yet- it's still a project to be done, not one that's already finished.
Bill and Hill's books are sitting by the MILLIONS in union controlled warehouses all over the country. It is a little tough to drive truckloads of them to flea markets and sell them to utterly disinterested folks. Methinks they are all destined to bypass the flea markets and be recycled as cardboard instead.
Bill and Hillary, on the other hand, are books that were bought out of 'obligation' to the cause but never actually read
I think you ate right. What self-respecting DIM won't have a copy of each prominently displayed on their bookshelf so that their guests can see them.