There is an interesting bug in iMessage.  Aparently some information survives a wipe and new users can still send and receive iMessages as the original owner.

These articles talk about stolen phones, but what about a used phone you sold or gifted?  Good Question.  What’s the answer?

Here are a few Articles on the matter that will get you closer to understanding.

Stolen iPhone? Your iMessages May Still Be Going To The Wrong Place | Gadget Lab | Wired.com.

Stolen, remote-wiped iPhones still get owners iMessages • The Register.