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Don't know about this Anna, but you could always post in teh Book Bazaar and simply list out the books you have that don't have ISBN's and just keep it updated.  How many do you have w/o them?  

I was just looking at Amazon and did more searching on their ASIN's.  Here is something I found on Wikipedia.com:

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The Amazon Standard Identification Number (ASIN) is a product identification number used by Amazon.com and its partners. Despite the name, it is not a "standard" outside of Amazon.com, but is used consistently for referencing products within the Amazon network of sites. The words "Amazon Standard" should therefore be read as "standard internal to Amazon", rather than "external standard initiated by Amazon".

Each product sold on Amazon.com is given a unique ASIN. For books with an ISBN, the ASIN and the ISBN are the same. However, owing to the fact that new ISBN-13 numbers will be issued, this is likely to change. Books without an ISBN and other products are also assigned ASINs.

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So while this may work, I don't know how feasible it is to use, since these are standard only to Amazon and may not be recognized by all.  What would happen if someone else had the same book as one you entered in as the ASIN and they used the feature of "posting without an ISBN"....things would probably get somewhat confusing.  Or maybe I'm making it too hard?  LOL

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