Debian (and, by extension, Raspbian) is comprised of free software. One of the core tenets of free, open-source software is that you should be able to modify and distribute changes to the code you have, so it is natural that you're allowed to modify Raspbian and redistribute your changes, under certain conditions.
Debian actually publish some guidelines on what you should do if creating a Debian derivative; notably, you cannot call your distribution "Debian" or in any way infringe on the trademark. I do not believe Raspbian is officially a registered trademark, but it would still be good etiquette to change the distribution name to avoid confusion and any doubt about the legality.
Note that Debian packages are distributed under various licenses such as the GNU GPL, BSD license, and so forth. If you've made modifications to these programs, generally, you will need to disclose your changes in public, or offer source on request. The terms depend for each license, and not all packages use the same license.
If you have simply used packages from the Debian repositories, you will probably not need to do anything additional to attribute the packages beyond the mechanisms already provided by Debian (such as checking licenses with APT).
See also: The Debian GNU/Linux FAQ — Redistributing Debian GNU/Linux in a commercial product.