I have become a big fan of the ebay sellers pangea_deals and certifiedusedphoneoutlet. They are owned by Assurant, the company that provides warranty replacements for the major carriers. Pangea lists "like new" phones: I recently bought a t-mobile note 4 from them for $320 shipped (lower than mint on Swappa) and condition was 9.5/10, flawless. Came with unused oem quick charger.
I also bought an S4 from certifiedusedphone for $115 (price for "like new" on pangea was $139). Condition was 9/10: perfect screen and only a couple of almost invisible scuffs on back.
Apart from the fact that these sellers offer the best condition at the lowest price, they have two other *big* advantages: because they are Assurant you are guaranteed a clean imei on t-mobile, not a trivial plus. They also have a 30 day satisfaction guarantee, no questions asked. Most ebay sellers that will take a non-defective return have a restocking fee. You pay shipping back to them but they sent me a usps label for $6, which was very convenient and reasonable.
At this point I would always check these two first, although I have had good dealings with Swappa also. Right now I am waiting for them to get t-mobile G4s. T-mobile is selling "pre-owned" G4s for $370 and I'm pretty sure they are getting them from this Assurant company. I'd expect the pangea price to come in at around $280 or less.
I have no commercial interest in these sellers; just a satisfied customer.