I've sold a few phones to Gazelle. Good prices (better than what the carriers would give me in trade-in value) and an easy process. I always exchange for an Amazon gift card, which gives more value and is super convenient.
I treat my phones really well, and have wondered, from time to time, if I could get away with calling their condition "like new" rather than just "good" on Gazelle, for the higher trade-in value. But I always go with "good." This last time, after I sent in my "good" phone, GAZELLE decided it fit the higher condition, so actually RAISED MY OFFER. This made me happy (more money, yay) but also made me think they're honest businesspeople.