How To: Prepare to Sell That Old iPhone
Make Sure the Phone is Backed Up
There’s no real need to start fresh with any missing contacts or apps when using a new iPhone. Follow our handy guide to setting up backups via iCloud or iTunes. Then, when the new iPhone is purchased, simply restore to the backup of that phone. On iTunes, this is just as simple as selecting the device from the list when choosing what to restore from. For iCloud backups, make sure to log in to the same iCloud account when restoring on the new device. It may be a good idea to note which iCloud account is being used on your old iPhone. Remember as well that backups are cross-compatible, so even an iPod touch backup could be used to restore to the new iPhone
Make Sure the Phone is Erased.
Sell Securely
A website like Gazelle or a store like GameStop may make it easy to sell the phone quickly and securely, but this will not net as much as selling it directly to another person will. Selling via eBay or Craigslist is a good way to quickly sell the phone, but it comes with the inherent risk of dealing with strangers. Hold on to the original receipt if possible in order to ensure that you have proof that you purchased it in case someone tries to claim that the phone was stolen. Especially log your phone’s IMEI or ESN, a phone's unique identifying number, if possible, as it is what the carriers use to track stolen phones. Here's how to find this information on the iPhone itself. Having a log of this may help if a scammer tries to claim you sold them a stolen phone.
Good luck out there!