With some professional iOS data recovery app, it can scan out all of deleted data and you can see what data still remains on the device and be sure that the files you previously deleted are no longer accessible or recoverable even with the latest software.When you plan to sell your phone, you never who will get it and what he will do with it. Therefore, you need to delete everything on your iPhone before selling it, for the sake of your privacy.
There are many methods in Google: http://www.safewiper.com/tutorials/erase-iphone-contacts-before-selling.html
I have tried factory reset on my iPhone 4S a couple times, and every time the process stuck on a grayscreen, and nothing completed in half an hour. When I rebooted the device, no single piece of my information was deleted. I am wondering how to fix that problem? Is there any alternatives to the default method? Input will be much appreciated.