Do I need ISBN for my ebook?
Whether or not you need an ISBN for your ebook depends on your distribution plans. If you are distributing your ebook only through your website via download, you probably don't need an ISBN. But if you plan to sell your ebook through other resellers, you'll have to find out what each website requires.
Amazon.com does not require an ISBN if you are publishing content with Kindle. The website says, "An ISBN (International Standard Book Number) is not required to publish content with Kindle Direct Publishing. Once your content is published on the KDP web site, Amazon.com will assign it a 10-digit ASIN (Amazon Standard Identification Number), which is unique to the eBook, and is an identification number for the Kindle Book on Amazon.com. If you already have an ISBN for your eBook, you'll be able to enter it during the publishing process. Do *not* use an ISBN for the print book edition."
BN's new PubIt! service takes a similar stance: "You do NOT an ISBN to sell your eBook through PubIt!. If you don't have an ISBN, just tell us that you don't have one by answering No when prompted. In that case, Barnes & Noble will assign an internal 13-digit identifier to your title for you when you submit the title to go on sale."
Apple's iBookstore, however, does require that you have an ISBN for each title you intend to sell.
Other sites, such as SmashWords, Sony, and others, all have their own requirements. If you plan to distribute through any of them, you'll have to check each site for instructions.