Bob, one thing that needs to be remembered is that lots of used-book sales are of books that are no longer in print. I recently bought a used copy of "Good King Sauerkraut" which didn't make me feel weaselly because I can't get it new.
I don't know, either, if I think that library sales reduce general sales. People who buy books, buy them, and library readers, in my experience, are either not interested in owning them or can't afford to.
I used to go to the library a lot, but in recent years I can afford to buy more books, and so I do. (And do and do and do.)
The internet probably makes it easier to sell secondhand books, but I don't know if it's all THAT much worse than before. Then again, I'm not a published author. Barbara might disagree.