With the advent of the Apple iPhone 7 announcements soon, PC Magazine shares important safeguards when selling your older model phone

http://www.pcmag.com/slideshow/story/301639/how-to-sell-your-iphone-safely

Selling your phone can reduce the price of a new one or mean a bit of extra spending money, depending on how you look at it. But before you pack up your old iPhone and ship it out, you should know there’s more to the process than just handing it over and waiting for cash to hit your bank account. You’re going to want to back up your current phone and wipe it of all traces of you. Privacy concerns abound with phones, from photos stored in iCloud to NSA spying, so clearing your phone of all data before you hand it over to its next owner is a must.

1. Back Up — The first thing to do is back everything up before you wipe it from your phone.
2. Unfind Your iPhone — Having Find My iPhone activated can sometimes block activation by a new owner
3. Sign Out of iMessage —¬† iPhone users who ditched their iPhone for another platform without logging out of iMessage were not receiving iMessages
4. Remove SIM Card — SIM card is the repository of your phone number, various bits of security data, and all the info needed for billing. It identifies you, your phone, and a lot of your activity on your phone. So you might want to remove it before you sell your phone.