NOT Amazon. I sent in a flawless 5s and it got outright rejected as defective (not downgraded). I've traded there successfully before but they clearly accept and reject based on not just arbitrary measures but their inventory. I expected a downgrade, a reject is simply an inventory decision, which is unacceptable when you are offering. The exact same thing happened to a friend.
Rant over. Craigslist will net you the most but be ready to sift through scammers and no shows. Cash is king. Gazelle is lower than most but dependable. Best Buy is more than Gazelle but might be a gift card.