Used books are hard to move. This is one reason why most libraries don’t take donations. These sales are for clearing shelves more than for making profit.
Not hard to move. Almost impossible to move. In my shelf I apart from some old leather bound books from my grandfather (Churchils world history, complete Hamsund) and CE and hard cover 40k books I have nothing but paper trash
People need to internalize this. Generally speaking, nobody who has a choice wants our old crap.
I use the Norwegian finn.no (listings of everything from old books to cars to houses. Norway is such a small market that one service covers all) It’s perfect for selling used (but little used) baby and child stuff. I have also bought a lot off Lego Duplo.

Speaking for myself, I’m too paranoid to buy a used book from some random charity because I can’t trust they ensure the books are clean before selling them.

My family brought home a bundle of music books from some charity event when I was a kid, and it unleashed an infestation of silverfish that proved impossible to get rid of. It’s been more than twenty years and they still pop up on my parent’s walls every few months.

A library I used to use would put old books for sale outside. If someone came in to pay for them, great, but if they happened to mysteriously disappear, that was fine too.