If the seller has a lot of phones listed (2 or 3 is "a lot" on Swappa), that means that the seller is probably buying large lots from companies that are upgrading, and - if you're lucky - doing some minimum refurbishment on them. Sometimes they work fine, ans any scratches on the screen are so small you'll never find them. Other times, the phone is barely usable.
If the seller is a private party, I've had decent luck - most people selling on Swappa take pictures at such an angle that the slightest ding looks as if the phone got run over, so what you get is actually a lot better than what it looked like on Swappa. But, as with all good things, people looking for another avenue of profit have ruined a good thing.
I buy locally now. LetGo, OfferUP and other sites like that (but not Crasig's List), and particularly, by getting to know a few independent store owners, who sometimes take a phone in trade when a customer upgrades (and can end up not paying much for the phone - the "price" is what they would have applied as a discount -, so you can get it at a decent price).