Bought an iPhone on Craigslist?

I purchased an iPhone 5 for AT&T off of Craigslist recently. I think it may be stolen though, I have no way of contacting the person I bought it from because now his account is gone, and it makes me mad because I traded him my iPhone 4S and $300 for it. My question is, since I'm an AT&T customer, and this phone came from AT&T, will I be able to use this phone, or will it carry a bad EMEI? I don't need the phone unlocked since I use the same service provider, so am I just screwed in the end, or can this phone be of any use to me on AT&T? If all else fails, am I able to find the guy who sold me the phone by tracking my 4S? It didn't have any find my iPhone apps on it.

If its reported stolen it will be blacklisted making it useless as a phone.

You will find out if it's blacklisted when you go to activate it. No worries, they won't treat you like a criminal. These things happen every day where people get sold stolen phones. Honest mistake, but you may be out your phone and your money.