First, be aware that Borders is going into liquidation his week which means they will no longer exist. I assume the website store will go away by the end of the week, so all of this might be a moot point, however this should not adversely affect KoboBooks.com.
Kobo uses a special eBook format which many around MobileRead have nicknamed kePub, short for Kobo ePub. The kePub has a special digital rights management (DRM) that keeps it from being displayed on non-Kobo readers. So kePubs will not work on Sony readers. For some books, but not all, Borders/KoboBooks stores have an Adobe ePub version which you can download to your PC/Mac and sideload onto readers that can view Adobe Digital Edition (ADE) style ePubs. Your reader has to have an ADE registration though, but this is easy to get/accomplish.