re: You have it in a nutshell.
I spend time at library sales (a lot of my old books end up there, too), and it's not hard to deduce the procedures Mary Anne describes from what you see on the tables.
I've never run into a librarian who let their political axe show (at least not where I could see it) although I doubt there's a lot of Republicans among them where I live. Now, book store employees are another matter...