16GB is not enough storage for any iPhone. And Apple has already stopped producing 16GB models from last one year. Apple is currently selling 32GB and 128GB model for iPhone 6s. Please choose at least 32GB model because it'll be good to have at least 32GB because iPhone 6s can shoot 4K resolution.
Today's Used iPhone 6 Plus Market Value Breakdown: The average resale market or trade-in value of a used iPhone 6 Plus is $61.77, with prices ranging between $48.00 and $72.00 from the best buyback stores.