If it's in mint condition with a warranty until October 2017, you're likely looking almost full face value. If the iPhone is out of the box and open, then approximately $500.00.
I have my mint 64GB 6s on here listed for $500. Haven't had any hits and I've got a warranty on it until August.
MacRumors is a little tough to sell on here. I haven't personally tried to sell anything on here yet. But I will say, if you are looking to sell it a little faster, you could you try Swappa.com.
Most have had success and it has a lower rate of dealing with those with fraud. I use to sell on eBay, then eBay has become an area where it's problematic with buyers trying to fraud the seller and filing fake claims, etc. Fortunately, I never had any fraudulent issues that some forum members have reported from eBay that Have been absolute horror stories.
I've had some luck here before. I'm really on the fence about selling it anyways so it's not stressing me just sitting there.
I've had the best luck buying from Craigslist. I meet in a public place, use whatever it is, then give money. Got an $800 laptop for $400 because the guy needed to sell quick to buy a Mac for his graphic design class. I got lucky.
Craigslist is tricky with iPhones though. Because you have to be cautious that the iPhone you were selling or buying isn't locked to a carrier under contract or stolen or where the potential seller could sell you the iPhone, then claim it as stolen, then Apple Locks the iPhone and then they get insurance money from their credit card company. UnFortunate, but those are accurate stories that actually happen to people.
I agree that selling on craigslist is lucrative and it's nice to have the cash in hand without dealing with the aftermath of eBay. Because when you sell on eBay, you have to deal with the buyer has six months to file a claim where they can potential say something happened or is wrong with the iPhone, then PayPal gets involved where they end up charging the seller the amount for the iPhone and debit your account.