There are valuable books and there are worthless books.

Worthless books include most fiction, self-help, and business
management theory.

However, many technical books are valuable and can be sold. It can be
hard to tell which are valuable and which are not, without extensive
research.

Three years ago, I bought four crates of Hyster forklift service
manuals, at an auction of a failed Hyster dealership, for $50 for all
four crates (plus buyer premium). To date, I sold 290 of them, and got
paid $22,349. I still have 236 of them, at the asking price of
$18,961, and I surmise that most of those will never be sold due to
lack of demand.

i