How to erase personal data from iPhone and keep apps?

I want to sell my iPhone and keep flappy bird on it. How do I do this but reset my phone to factory so my personal data isn't on it?

Not possible, the app is tied to your Apple ID.

Also note: There's no point in selling it with Flappy bird. It doesn't make it more valuable.

The people who are listing their phones for $1,000s aren't getting actual buyers (they all have either 0 bids, or bids from fake accounts)