Report the sale / seller to the police. More than likely they are scammers and reported the phone lost or stolen after the sale to get insurance money or a free / cheap replacement. That is why you should never buy a phone from Craigslist or anywhere else without having the deal completed at an AT&T where they can verify the IMEI and owner or at a Police station where a record of the sale can be referenced if needed. Of course, it is too late for you now. Doing anything to a blacklisted IMEI is illegal and is not condoned or discussed at XDA.
SGH-I337 (AT&T) 32GB
P-ROM (1st Gen.), De-Odexed, AMDL
Auto-LOKI OUDHS CWM
Make sure you read and understand the "PINNED" threads in the "General" and "Q & A" sections of the AT&T S4 forum BEFORE you ask for help. Use the forum's "search" function to find posts with additional information. You should not expect others to help you if you haven't tried to help yourself first. Also, read, understand, and follow the "XDA" rules.