If your device has come out on the other side without any noticeable wear and tear, you're in for a big payday. The above payout ranges are estimated based on phones listed in "good" condition on all of the aforementioned sites. That's why you get a phone case, people.
Trade it in for store credit
Where to go: Apple, Verizon, Sprint, AT&T, T-Mobile, Best Buy, Amazon
iPhone 5, 5C, 5S: $90 - $300
iPhone 6, 6+: $325 - $375
Most wireless carriers have buyback programs where customers can trade in their device for money towards a new one (or a gift card), though the amount varies widely based on the condition of the phone. The bulk of them require you to send it in a prepaid shipping envelope to be appraised by their experts, who'll apply payouts once they've decided how much it's worth. If you'd prefer fast cash, Apple and certain Best Buy stores will appraise it on the premises and offer to pay you in credit or in the form of a gift card on the spot.