I don’t see the need for a used phone when you can buy a great unlocked phone new for $200 or less.
The specs on that new $200 phone are unlikely to be any better than many used phone in “excellent” condition for less than half the price. The last used phone I purchased for my wife was directly from Freedompop, a Sprint MVNO. It was a “groupon special” for $59 with shipping for a previous Samsung “flagship” and does not have a contract. It does not have a mark on it and no problems that we have found, and better specs than most new $200 "unlocked" phones.
Personally, I trust many of the sellers on eBay more than I trust Freedompop. People who have used Freedompop probably understand what I am talking about; they do have some pitfalls. But for a spare phone that spends most of its time in a glove box... the basic plan doesn't cost anything per month as long as you stay within your free minutes, free texts, and free data. You just have to be careful when signing up not to jump at any of their “free limited time” offers. They do have occasional “special offers” that are hard to pass up. So I have to admit that I am paying a couple dollars a month for “data rollover” on a couple of accounts and unlimited text and voice on one of them for $5 a month. They build up data for cheap and I use them for listening to Pandora on the car radio. I use another MVNO for our primary cell accounts.