The epub format is universal. If you create a fully functional, validated epub, any reader can read it (including a Kindle Tablet and/or iPad). And you can use the epub to create an Amazon mobi file via their KindleGen converter. The epub file I submitted to become a Nook file is the exact same epub I submitted to Adobe Digital and other ebook retailers. You can sell the epub file anywhere that accepts that format, including off your own website . The key here is creating a single perfect epub that can function on all these readers. It's not difficult to do, but you need to know what you are doing.
When you list your book for sale through PubIt on Barnes and Noble, it can be read on a whole list of devices like the iPad, iPhone, Android Tablets and Phones, as well as desktop PC's and Macs by using the Nook app. Anyone who has purchased your book on Barnes and Noble would be able to see your book in their library using this app. You could also do as the previous poster suggested and distribute just the ePub file to people with other devices but then how would you be paid?