Changing carriers

Ok, about 2 months ago, I bought an Iphone 4s through my boyfriends Sprint plan. He walked out and told Sprint my phone was lost or stolen and he wont turn it back on. I would like to take it to another carrier but was told by Cricket that Apple doesn't allow it at all. They said if I unlocked it and they hooked it up, then Apple would shut it off. Is this true and how do I change carriers? I know I could possibly sell my phone but who is going to buy my sprint phone and how do I sell it? Thanks. I looked and only saw ATT and Verizon questions not sure if that solves my problem.