Monetarily, most ex-library (ex--lib) books are not valuable; but they are readable

 Ex-lib books are often discarded from libraries and picked up/ purchased by booksellers for sale.  However, some collectors buy them at low prices, and read them; others use them for research or specialty book collecting.

Over on an eBay community forum "gtb-books", a book scout, comments:

‎02-09-2013

Former library books are seldom worth anything money-wise, unless they are very old, and in very good or better condition.  Otherwise they are just reading copies.  Collectors usually want pristine  firsts [first editions], especially of more recently published works.

(https://community.ebay.com/t5/Archive-Booksellers/Ex-Library-Books/td-p/2738919)

Many book collectors insist on having fine condition dust jackets or covers on their books

  Generally speaking,  a hardback book's dust jacket is an asset when shopping; the price for a book is higher if it has a book jacket in fine condition.  A tear or other problem on the jacket lowers the price of the book; many used books sell with no jacket, which reduce a book's value significantly.

According to Bibio, 

 Once intended to be temporary and disposable protection for beautifully bound books, dust jackets have become–in some ways–more valuable than the books they protect.

Biblio, a book search engine, also has helpful articles on book collecting.

(https://www.biblio.com/blog/2015/04/a-brief-history-of-the-dust-jacket/)

(https://www.biblio.com/blog/2020/10/how-do-i-find-a-specific-book-cover/)

Care to buy from smaller booksellers than Amazon?

 Try Amazon's Marketplace.  Whenever you shop at Amazon for books, you are given the option of deciding from small sellers, mostly based on book condition.  These choices are provided by  independent sellers.

Amazon says that about half of all of its items sold come from their Marketplace program.  Shopping there may bring you the best price for your book.

(amazon.com)

A virtual book fair is on March 4 -March 6, 2021

 You can shop at a book fair online, from the comfort of your home.  From the international League of Antiquarian Booksellers (ILAB):

the Antiquarian Booksellers' Association of America (ABAA) has announced a virtual book fair, starting 4 March. ABAA members and ILAB-affiliated booksellers are allowed to exhibit. 

 (https://ilab.org/events/california-virtual-book-fair-abaa)

Buying used books from a nonprofit

 BetterWorldBooks is a nonprofit used bookstore that gives to charities.

Plus, for those who would like to donate their own books, they have physical drop-off boxes; if one is not in your area, you can mail them to one of several locatiions.  *Remember that books can be heavy shipments, and BetterWorldBooks have mailing restrictions specified on their donations page.

(https://www.betterworldbooks.com/)