Sharing my paid app on my iOS devices
When you purchase the app, it stays associated to the Apple ID you used. You can install it on other iOS devices, too.

Quoth Apple:


  You can associate your Apple ID with up to 10 devices, with a maximum of 5 computers at any time. Each computer must also be authorized using the same Apple ID. 


You can re-install it on a new iOS device in the future. Remember to Erase All Content and Settings, and sign out of iCloud before you sell your iPhone 5.