Used iPhone 5 - What do I need to do to register as the new iPhone owner, have warranty, etc
The warranty is established by Apple based on the serial number and nothing else.

If you want to register things, you can do several things:


Contact AppleCare http://www.apple.com/support/contact/ or via phone
Self register at https://register.apple.com/cgi-bin/WebObjects/GlobaliReg.woa
Self register by erasing all content and settings (via Settings App - Reset or iTunes restore) and opting in with an Apple ID to register the device.