Upgrading your mobile device? It has become standard practice to upgrade to a newer device every one to three years. And when doing so, people often sell, donate or discard the old one. The goal is often to sell the old one to make up the difference in cost for the new one.
http://robertsiciliano.com/blog/2013/10/24/selling-your-smartphone-beware-of-stowaway-data/