I want to purchase a used iPhone 4 on Craigslist that originally belongs to Sprint. I would get the iPhone 4 for T-mobile, but T-Mobile only has iPhone 4s and 5. I hope to unlock the iPhone 4 for T-Mobile, but not if it's illegal. I want to know if it is illegal to Unlock a used iPhone 4, so that's where you guys come in. I would really appreciate it if you guys could help me out with my little problem.
(I HOPE YOU GUYS KNOW THAT I'M NOT TALKING ABOUT JAILBREAKING. JAILBREAKING AND UNLOCKING ARE TWO DIFFERENT THINGS)