Anirvan Chatterjee Bookfinder weblog recently wrote me a private email explaining why ISBN-based book searches have their limits at online booksellers:
Isbn.nu and many other similar sites take an ISBN (i.e. an SKU), and report back on the lowest price for books with that ISBN at various locations online. But books are complex, and have attributes very different from other products one might choose to buy online via the SKU.
For one thing, books are often available under different ISBNs; the same book can often be available under three or four ISBNs in the US (hardcover, softcover, mass market paperback, movie tie-in edition, school edition, etc.), and dozens more around the world. On top of that, a large proportion of the used books for sale online aren’t tagged with ISBN metadata by the sellers, making them unusable by most systems, which can’t handle non-ISBN book data. This means that typical ISBN-based book shopbot tools are searching a relatively limited universe of items.
Later he mentions a little known fact about ISBNs:
Things are further complicated by the fact that you can retroactively apply for an ISBN, so you can have a non-ISBN-marked
book (or ebook) suddenly acquire an ISBN after you buy it.
Also: the new expanded ISBN was implemented to keep track of J.C. Oates rapid publication schedule (just kidding!). I had lunch with J.C. Oates once btw.