I don't even know why we are discussing gazelle here. Like cashforiPhones, it's an online pawn shop. You'll get no where near the actual value.
Here's what they do:
1. Offer you $180 for an item that is worth $400 (a lot of people think that's a good deal since they only paid $200 for it and don't understand subsidization).
2. Email you after receiving the item and tell you it's not worth $180, because it's scratched, scuffed or whatever, and offer you $130.
3. Hope you'll accept the lower offer rather than asking to have it shipped back to you (most people that chose this service will accept since they've already made up their mind that it's an easier option than eBay or Craigslist).
4. Pay you $130, sell it on eBay for $400 and make $270 profit (around $240 net profit after factoring in PayPal, eBay, and shipping fees).
If you want to maximize your profit, just put it up on craigslist. iPhones sell themselves. It's not difficult and it doesn't take long.