BBeB (or LRF/LRX) is the format Sony sold up until this year, they now sell books in the more universal ePub format with Adobe DRM (they upgraded all/most of folks previous purchases so they can be re-downloaded in ePub too).
The newer Sony readers can do both formats, but the 500 needs to be sent in to Sony for a firmware upgrade, however doing the upgrade means you lose the ability to view LRF/LRX (BBeB) format books. Assuming they're still doing the upgrade that is. I'm not sure if there was a time limit or not, the offer started around November '09.
Some links that may be of help...
https://www.mobileread.com/forums/showthread.php?t=81198
https://www.mobileread.com/forums/showthread.php?t=62272
https://www.mobileread.com/forums/showthread.php?t=62252
https://www.mobileread.com/forums/showthread.php?t=63278
The other option would be to remove the DRM from the ePub and then convert it to LRF using Calibre so it will work on a non-upgraded 500.