Not aure about the authors, but I buy used books often. I buy the large majority of our books through ABE Books, it searches out many used book sellers and I rarely pay more than a couple dollars for books, but some go directly to the book dealers web pages because they have specials such as buying a certain number of books and getting one free.
Almost all the books I buy are in good condition, but are old library books with labeling to match. That does not bother me, but does make it look like I forget to return library books, haha. Sometimes it is really fun, some books have the old library card in tact with names and dates listed. My kids think it is fun because new library books do not have cards.
So far the books have arrived as descriptions stated, so if you read the descriptions you should be able to find books that match your needs. One time a book arrived with missing pages, I contacted the seller and was refunded very quickly. I have had great success with all the book sellers on ABE Books. I have even received books from over seas sellers with success.
Our library continues to grow with hopes to pass down to my kids, as my mom passed down to my siblings and me.