Originally Posted by Ki

-Factory reset it to stock (2nd most important)
-Double and triple check that your bank information is no longer accessible through your system. (Most important!!)
-Any users associated with it are deleted.

You basically want to get back to how you bought it as much as possible.

With my Wii U, there was a way to factory reset the entire system.

This should help you: PS3 - How to do a Factory or Hard Reset