He surely could do it. As many other have before. But he forgot to factor in two important things.
1.) sell the phone, stay with sprint, keep the current phone enjoy the profits
2.) sell the phone, leave sprint, use the profits to pay off the ETF and price of the device aka WHATS THE POINT?? All, if not most, proceeds go to paying off the dumb idea and the OP is back to square one. No extra cash. No sprint plan and no new iPhone.
OP, did you really truly honestly think that sprint would send you a new iPhone, for $500 off retail price, for you to sell and then cancel your plan with no course of action? ummmm they're gonna want their phone back or you will have to pay the difference for it. You were sold that phone at the discount on the grounds you sign another contract extension with them. I mean seriously?