I'd say Gazelle would be great for a quick sale, but I believe the OP could possibly get more money from craigslist or even eBay. With eBay however there's scammers and I'd not recommend that option.
forget about gazelle, you won't get very much money from it. I would sell it online, I don't like ebay so I usually sell on iphone forums or cellsolo.com
gazelle is a rip off. verizon is offering cash for old iphones regardless of carrier. but the good old fashioned way of selling on imore.com has still led me to MUCH more profit than these other sites that are just resellers. you cant sell to them because they will lowball you so they can in turn flip it for profit. i would recommend any classified section on any forum is the best route.
Easiest, quickest way to sell would be Craigslist. If you're patient, the right offer will come. If you've sold online in the past, I'd recommend Amazon or eBay; like other posters have said, be wary of scammers.
Easy and quick are key but I dont want to get ripped off either. Both Verizon and gazelle were my first options but they don't offer too much. I'll search around on Craigslist. I'd try to sell in the marketplace place on forums but seeing as I'm decently new I don't have enough posts yet
If its in great condition and you have all the stuff that came with it, walk into Bestbuy and they'll give you a giftcard worth about $400 for it. Then stroll on over to their Mobility section, giftcard in hand, and walk out with the phone you wanted.