I got an 64GB AT&T model in very good condition for 399 on eBay. I think that was a steal but I wouldn't have paid more than 450 for a 64GB model. If you want a quick sell you can probably sell it for $350 locally on craigslist. Online you can list for more (everyone does to account for fees) but that's exactly the problem....fees and a lot of competition.
It really all depends on your location. When I was in Delaware it was easy to find buyers for iPhones and high end Android because there wasn't a lot of people selling. But now in Charlotte, there are plenty of listings for these devices. And anyone that knows economics knows that when there's a surplus, prices go down. Another thing is that after a year, it appears that many people don't care about the device being it mint condition as much. So while sellers definitely like to tout this as a selling point (as they should) but it's hard to sell it at a premium because of that. Sure they would like mint condition but they would gladly pay $50 less for a device with some wear that works perfectly when all else is equal. Last thing that affects you is Sprint. That's both good an bad. The bad is that is limits the number of users because Verizon network is larger and AT&T and Tmobile ones can be unlocked so it appeals to more buyers. The good is that for Sprint buyers, it could be harder for them to find a device for those very reasons so you could actually end up selling for more than an AT&T user because the Sprint buyer has to pay more as they have less options.
So having said all of that (sorry for the long post), I would like online (Here, swappa, eBay, Amazon) for around $400. Your goal should probably be to get no less than $350 after any fees and shipping for online sales.