I especially like second hand book sales at libraries/estate sales/etc because it totally derisks the purchase. For two bucks I won't feel guilty if I don't finish it. By the same token, though, I try to honestly look at my shelves periodically and ask 'am I gonna read this' or 'am I ever gonna read this again?'
Local libraries are a gold mine. Some of them resell books that were removed from circulation for like $1 for paperbacks, $2 for hardbacks, etc. I've gotten tons of books from those sales.
There's still the guilt when you have to tell the movers that this big stack of boxes is filled with heavy books.
I get rid of any book that can be found used for under a couple bucks or so used online, unless I am actively reading it or need it for something. I can always get it again later when I feel like reading it, just bookmark a link to the place it's sold.
Why guilt? No reason to feel guilty for anything.
I collect old books and buy anything that I can find pre-1950. Last time we moved I had 2000+ books in ~15kg boxes. No guild or shame about it. I love my books.
