Ouch. I hate when I hear from customers that they have bought a phone off of craigslist. There might be a way for you to get your money back, or even get the previous owner to unlock it for you (since they have nothing to gain from selling you a device that doesn't work unless they stole it, it's probably an honest mistake) for future used phone purchases, always go for EBay, if you simply must purchase a used phone from a stranger, because they have an excellent customer care system. And after purchasing a phone from anyone that is not an authorized reseller, get the phone checked out at an apple store IMMEDIATELY. When buying this sort of thing, you always want to be aware of your warranty situation. The previous user may have made unauthorized changes to the hardware, voiding the warranty and possibly having done shotty work. But anyways, my advice here is to always purchase from apple or a carrier, most carriers sell used phones and it's a lot easier to get your money back from them.