Just to add to this post. Be sure not to use this method on a jailbroken device. It will get stuck at the boot logo and a full restore in iTunes will then be required. This method is faster than a restore and perfectly fine for stock devices.
I think you misunderstood the context of my reply to her. I meant that the app she posted isn't going to do anything a restore wouldn't do.
I realize the FBI could probably retrieve the data after a restore, but I'm not going to fuel any paranoia here. I don't mention that when people ask how to wipe their phones because most people don't have top secret/classified information on their phones that the FBI or CIA wants. It's not something the average person needs to worry about when they sell their phone on Craigslist. And if you are concerned about that, you're better off just destroying the device. It won't be worth the $300 you'll get from the undercover agent who's waiting for your Craigslist ad.