That's a pretty good price but be careful since Gazelle can sometimes decrease your offer if the condition is not the same as described. If you can do craigslist, that could be better.
You could probably get at least $100 more on CraigsList but look at that $100 as a price to pay for not risking your life meeting some random potential creep wanting to buy an iPhone. This is why I sold my 5s on Gazelle.
Sold my Gold iPhone 5s 32gb Verizon for $450 ... the average pricing you're going to get from a Gazelle type company will be $200-350 because they know they can get $400-550 easy. I low balled my price to sell quickly but just about everyone on CL was charging $550.
selling on CL is risky, meet in a public place with cameras and a security guard. I always meet in the inside door my local Best Buy since they have cameras and security officer a few feet away.
You'll always get more money on eBay, but it just takes forever to get your money waiting for the seller to leave you positive feedback before pay pal releases your money. I've used Usell.com a couple times successfully and they usually give you more money that gazelle.