I haven't purchased ebooks through Barnes and Noble, but I understand how annoying it can be getting all the ebook stores/ecosystems/formats/devices to play nice with each other. It was the reason I continued to buy paper books up until recently.
My solution: buy your ebook in any store that allows downloads (Amazon, Google, Kobo), and import into Calibre. Change settings to automatically convert imported ebooks to .epub format if they aren't already (the DRM-stripping plugin is optional ;-)). Set your library folder to dropbox, and voila, you have unfettered access from anywhere (and transferring to your Kobo Aura is easy too).