There are only two places I know of online that are trustworthy for buying used devices.
https://www.gazelle.com
https://www.macofalltrades.com
Problem is, their used prices aren't anything great (sometimes more than Apple's refurb page), and you get what you get. We bought a couple of used devices from Gazelle years ago, and even the "Excellent" condition priced ones had noticeable wear. Particularly a pink screen on one of them from being swiped thousands of times.
You won't get that kind of thing from Apple. Their refurb prices are a bit on the high side because they'll do whatever it takes to make the phone "like new". New case, new screen (if necessary) and always a new battery if it's a bit older model. Also, Apple's refurbs are always unlocked, and so can be used with any carrier.