When looking for a textbook online how important is the ISBN #?

I am looking for ISBN # 1418004553 and 13 978-0-323-04937-5 online becuase it looks to be less than a third of the price of what the school is charging. I am looking for Fundamentals of Anatomy and Physiology 2nd edition which I found, but the ISBN does not match what I was given. Is it the same book?|||It's probably the same book, just a different edition. I think the ISBN is the identification number.