You can check out isbn.nu. It does a comparision of 10 online booksellers. Since you're looking for entering multiple ISBNs, I think it would be easy to write a small webservice that feeds isbn.nu these numbers and gets back the results. I remember doing this something like this before, but for the life of me, I cannot find that code :) Since the url goes something like
http://isbn.nu/0001024671 where the number is the ISBN, it should be hassle-free.
