There are many book sales in the world. Recently, I went to Big Bad Wolf sale, Big Bad Wolf sale is an important book sale in Malaysia where the book are priced at below market price at an average of RM10 per book.
There are two advice I would give to potential buyers of such books at book sale. First, buy only good books that you know. This is a no brainer, you want good books. How do you know which good book to buy? You need to inform yourself of good books by browsing online, such as Quora. There are many sub such as this. Or, you can browse reddit, or New York Times Bestseller list here.
Second, if you have no clue at all which book to buy, find a good book by seeing if the book is a bestseller. This is easy, search the cover of the book for the word “Bestseller”. That is, buy best seller if you are unsure what to buy. A bestseller is a book that sold a certain amount of copies, which guarantees its reputation and quality. There are few books that I gotten by using this method such as Gulp by Mary Roach.