The mp3's you "bought" will (most likely) not play in rockbox. Mp3's from online stores such as itunes and Napster contain a type of encryption that is commonly know as Digital Rights Management (DRM).
This is stated in the license agreements you agreed to when you singed up for itunes store, Napster etc. It is basically a software encryption that restricts your usage of the mp3, it states whether or not you can copy it, burn it to CD, or play it in programs besides itunes, among other things. This means that since rockbox cannot hold a DRM license for itunes mp3's it can't play them (and I severly doubt Rockbox will ever play and DRM'd music).