Iphone 5 activation and ESN?

I bought an used iphone 5 through Craigslist for relatively cheap.

It's not perfect cosmetically, but fully functioning. When I was buying this phone we had some issues, well, I think I had some issue with the seller. He seemed not that trustworthy; however, I ended up buying it since verzion staff told me that this phone has clean ESN, and is deactivated. So I activated the phone with my number, and it is working right now.

Although he said it's his, but what if it's a stole phone, and THE REAL OWNER reports as "stolen", What is the outcome? And what could happen to me?

You would have to give it back of course. You would not get in trouble though because it was a legitimate purchase thru a paper or other source. The seller now would get in trouble but not you. I wouldn't worry too much though because if it hasn't been reported stolen by now it probably isn't. When stolen the ESN number is placed on a blacklist and they can't be activated. Thats all!