I refuse to trade in my 6 Plus for this but seeing as they are saying "Any working smartphone" i'm going to purchase a no contract prepaid phone from best buy for $15 and trade it in. I looked on the buyback program and it is currently worth $3 from sprint for trade in so i'm going this route.
UPDATE: Sprint told me they will accept the $15 prepaid smartphone and she pointed out that it was "4.5 inches" which qualified it. Its the Verizon Wireless Prepaid LG Optimus Exceed 2 LG-VS450PP and its available with no contract at best buy for $15.
4.5" ?!? If that's the criteria then that excludes the iPhone 5s even, which I'm sure they will accept. Sounds like some BS to me but I'm glad you found something that works for you.