Where Can i Sell my iPhone 5 (3 Weeks old) with Flappy installed?

Black iphone5 16gb with flappy bird. Email to make offer maximilian-bramsiepe@gmx.de from Germany

You should try to sell it on eBay or Craigslist instead of advertising here.

Makes no difference if Flappy Birds is installed or not, for some one to use it they would need to have access to your Apple ID, the e-mail you registered your ID with and password, but the Transfer of Apple user accounts in not permitted.
You could sell the phone on eBay, just advertise it as a iPhone 5.

Makes no sense since you would have to provide your Apple ID's password for the app to work since it was bought under your own.

£0.01
And that is for the phone…

You can give add for your iphone just free in sell and buying sites.