How do I transfer ownership of a used iPhone?

If you are the buyer, insist that the seller unregister the phone with Apple so it isn't tied to his/her Apple ID (which could later cause problems with iMessage). To to this they need to:

1. Go to: https://supportprofile.apple.com/MySupportProfile.do
2. Log in if not already logged in.
3. Click on ">" to the right of the phone listing
4. Click "unregister"

Maybe they should do this in front of you so you know it's no longer registered to them.

Well there is not title document for the phone as such. For a car you would have registration or ownership papers. I guess if you had the original receipt from when you purchased the phone that would be sufficent proof to the buyer that it was yours to sell but other than that I am not sure what else you could do

I just sold my old phone and went to unregister the phone when I thought about something. Would the applecare purchased for the phone be void with me unregistering? Or does it stay attached to the serial number of the phone itself?

see https://discussions.apple.com/thread/2683817

please tell me that how can get the restriction pascode,

This info ***** big time. I have 3 iphones, and several registered products and only 5 shows up. No iPhone whatsoever.

Applecare stays until you call and cancel