I used to use a store like that in Dubai, they had a system where the book you purchased already had a stamp in it, when you returned it you would get something like 5 dirhams back and could get it deducted from the price of another book,but you couldnt return 4 books in exchange for one book that cost say 20 dirhams. I used the store a lot and even just donated some of my own paperbacks rather than just giving them away to somebody who may never read them . I liked the thought that my old books could have a life after me . The shop was very busy.