I've read a few articles about selling your 1-year old iPhone to help cover the expense of getting a newly-release iPhone without the new contract price, but I'm still unclear about my situation. I got a 5S at release with a new 2-year Sprint contract. What's the best way to sell it? I'm thinking I should upgrade to the 6 at release, and then sell my 5S after. That way I won't be without a phone in the meantime. And what about the whole no sim card, unlock, etc stuff that is always discussed? Do I need to have Sprint do something to my 5S after I buy the 6, or is it automatically no longer linked to my account and fully usable by another Sprint user? If anyone knows what a last gen iPhone (Sprint) in great condition would sell for, let me know.