I sold my Kindle, an Amazon Dot, an iPhone and an iPad mini to Amazon in exchange for an Amazon gift card added to my account. Amazon offered me the most money for each item compared to Gazelle et al.
I have a relative who’s an ebay maven with a very high reputation feedback score, and if I have something Amazon won’t take, or is a more specialized sale (like an art book) I’ll occasionally sell through her, as established sellers with good reps get higher prices than people who aren’t active with a track record.
In the past I’ve sold camera gear on Craigslist, usually meeting people at home or a local Starbucks. Tons of horrible, horrible cheap people on Craigslist making insultingly lowball offers, but once I’ve found someone interested who’s made a fair offer/counteroffer, the deals have gone smoothly.