thanks @point2make and @Rev Bruce Jackson for answering my question. as for me, i have established some kind of a rule. as i am fond of reading some categories from current affairs, i buy most of the books from the bookstore because current information is important. bookstore are up to date in releasing recently published books. but seldom do i buy hardbound. i could wait a little more for the paperback edition.
however, for those books that are not affected by the passage of time and information like novels, and non-fiction classics of famous contemporary authors, that one i would buy from discount book shop. also picture or coffee table books.
i established this rule because i have bought several books from discount shops that were more than 3 to 5 years old. some i did not enjoy reading as much because i am looking as far back in retrospect. you agree that sometimes there is a joy or an AHA! moment when you read a book and truly understand the situation as it is happening? that's what i am after... (,")