I am unable to contact the owner, he seems to be disappeared.
I also checked with AT$T this phone is clean and not blocklisted. also i am not getting any loosed message.
call apple and give it a shot why not. Else hold on to it. There may be a jailbreak, hack or something, I doubt but its better than throwing it away.
Call/Email the craigslist person. If it says it must be unlocked by the person. Email that email address and see if you get a hit back.