Unless you plan on selling at a deep discount, no one's going to want to buy the phone from you when they can get a new one in less than a week - roughly comparable to the time it'd take to get sent to them from you - from a reliable source.
Maybe some non-US buyers might, but, then, you'd be dealing with an overseas sale, and those can be dicey if you don't know the buyer. So, taking into account the aforementioned termination fees, monthly fees, and misc fees, you're basically going to end up eating the entire cost of the iPhone.
Unless you sell to Nigerian buyers. They're filthy rich and always in need of ways to get their money out of the country.