Many of the older books do not have ISBN numbers and many have more than one number. This system was adopted in recent years. Books from and the sixties and seventies do not have the number of the back cover. It might be on the title page inside the book. You are limiting yourself to only recent editions of the book. When I post book to sell I try to use the ISBN number to get more fill information for my ads. Half of the time I can't find anything or receive multiple hits. If it is very popular books there may be dozens of correct ISBN numbers. If you are looking for a specific edition it would be valuable. This is true for school text book each edition has a different number. Fiction books are really the same regardless of the year it was published.  If you want to find a cheapest or best book for the price and shipping use the name and or author.

Also do a search in the store inventories using the advanced searching feature just below the regular search box. Often the stores will have better prices. Be sure to look at the shipping costs before you buy. Sometimes you can be surprised in the range of shipping costs available. Last night I was looking at a book for 25 cents but the shipping cost was $8.95. The low price was misleading.

Also if you are buying books see if the seller has a shipping discount for multiple purchases. You can save lots of money buying several books as once. It saves the seller expenses in paypal fees, packaging, and increases their sales volume. If you are not sure about something ask the seller. It always better to ask questions before you hit the buy it now or bid button.