Returning iPhone

I recently bought an iPhone, when I got it I logged into my apple ID and it worked a couple of hours later the iCloud was locked and I couldn't use it. I can't get a hold of the person who sold it to me. I'm not sure if the person who sold it was using me to commit insurance fraud or if they sold me a stolen phone. How can I get a hold of the original owner because I would like to return it if it was stolen or try to get my money back if the person who sold it to me was commiting fraud,the phone is useless to me.