Now is the best time to sell your iPhone
Whatever your situation might be, if you're thinking about selling your iPhone, you might want to pull the trigger sooner rather than later.
Every time Apple announces new iPhones, the sellback prices of older iPhone models drop considerably - sometimes by $100 or more - across most sites, from Gazelle to eBay and the like.
If you need a good place to sell back your iPhone, you'll find plenty of options. You'll probably get the most money from eBay, but several other sites like Gazelle, Glyde, Swappa, buymyi, and even Amazon make it incredibly easy, as most of them will offer to send you a box and a label to ship off your iPhone for free, and either pay you via PayPal, Amazon.com gift card, or a check in the mail just a few days later.
Keep in mind: The new iPhones will likely be announced in the next few weeks, and released shortly thereafter. So if this iPhone you plan on selling is the main phone you use, make sure you're going to be okay if you're phone-less for awhile, or just have a backup phone ready.