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I use craigslist.

Anyone looking to sell a 5s? Send me a private msg. I'm on ATT.
 
I'm actually surprised by how many people sell on CL. My wife and I sell household stuff (furniture, baby gear) but I hate it because CL people are flaky as hell.

I have sold all my phones on eBay but I'm sick of the fees and it has become just as much of a hassle. I'll sell my 5S this year on Swappa or Gazelle.
 
If you'd like to avoid eBay I'd suggest using either Craigslist (immediate cash payments, no need to deal with shipping) or Swappa.com (no annoying multiple seller fees because the buyer pays the $10 fee which is added to the price they see). I've done both methods personally and I've been happy with both.

For Craigslist just make sure to follow the basic guidelines of meeting in a public place, meet during the day, accept only cash, and either tell a friend where you're going to meet the person or (preferably) take that friend with you and have them sit a couple of tables away from you so they can keep an eye on you.
 
Best BuyBack

If I were you I would check out Buybackplus.com. They will offer you 20% more cash than the other major websites. Good luck!
 
In no particular order
Craigslist, Ebay (though I avoid ebay at this stage), Gazelle and trading it into VZW
 
I used to do ebay but it seems like too much of a hassle now. I traded my 4s back to Verizon and got $210 Verizon gift card. I used that to pay my bill for the next couple of months.
 
Craigslist or social networking for selling my devices.

As for buying: I am purchasing unlocked variants directly from the manufacturer.
 
Hassle v. Price

I've used Gazelle to sell a number of old iPhones, but mainly because the process is quick and painless. The price was lower (to much lower) than you would get on eBay or CL, but there was zero hassle. I sold my old iPad 3 to a friend, for double what Gazelle was offering (but far less than the Apple refurb price, and mine still had 6 months of Applecare+ on it). I'd rather avoid the hassle of selling on eBay or CL, so using Gazelle was fine, but if you can sell an iDevice easily to somebody you trust, that's best.
 
Sold my last one on Facebook. My area has a group that is essentially an online flea market. People post random crap for sale or are looking for. Had it sold within 10 minutes of posting it. I like it better because it's more localized than craigslist as I don't live in a major metro area.
 
I've noticed that the sell-your-stuff sites REALLY screw you over hard if you have a higher capacity device.

On one site, the sale value of a 64GB iPhone 5 is only ten bucks more than the 16GB version. Ten bucks for what was a $200 difference new!

So if you have a 32 or 64GB iPhone you'd be best off selling on eBay or Craigslist or some other place where you may find customers wanting to pay a bit more for the increased capacity.
 
I find it interesting that the OP said he didn't want any hassle and just wanted a buy back site, and then everyone suggested craigslist and ebay. Both those sites are full of hassles. I haven't sold on ebay in years, and every time I sell on craigslist I always get stood up at least twice when arranging to meet someone.

If you want the ultimate in hassle free, try GameStop. I haven't tried it myself, but I've heard good things and the prices they offer are competitive with Gazelle. The difference is you just take the phone into any GameStop store, the manager inspects it, and you walk out with cash. No need to wonder if your auction will meet the reserve price, no need to drive across town just to have some stranger not show up, no need to even find a box to ship it in. Yeah, there are ways to get more money, but this lets you just be done with everything and move on as fast as possible.
 
eBay, Swappa.com, or CraigsList. I hate using C-List though. Ive been literally robbed in a face to face transaction and dont trust anyone from C-List typically.
 
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