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ISBN is pretty useless on Amazon. Since it's a closed system their own asin nr. should suffice. Since ISBN use is to uniquely ID a specific product for every distribution channel (Bookland EAN) it might have sense to have one for same editions sold via multiple sellers (e. g. have one for your epub and another for your PDF - both self-made and offered through multiple shops) This way it's clear that regardless if the book is bought in shop A or B it's the same file. But as amazon is concerned: the only place to get amazon books is amazon. Hence no need for an "external" identifier. As for Slushwords: It's their ISBN for their edition. No use of it for another.
If you want your own ebooks with ISBN you need to buy your own.
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