I sold an old iPhone 4s and it was attached to an old Apple ID tgat I can’t remember any of the log in info and the email is deleted. How can the new buyer register it under his Apple ID.

If you can't remember the account's password : http://iforgot.apple.com

Or, and if you were the original purchaser of the phone (i.e. it wasn’t bought used), you could try going to an Apple Store (make a Genius Bar appointment if you do) or try contacting iTunes Support online (http://www.apple.com/support/itunes/ww) and see if they can remove the phone from your account - they are likely to want to see proof of ownership before they might help e.g. original receipt with the serial number on it