I bought Iphone...

Hi apple community i just wanna share my story I bought Iphone from kijiji yeah its working properly only the problem is screen cracked everything is good so i pay for it cause yeah its my dream phone so i pay right away and then when i came home the phone remotely shutdown by itself and when the phone turn on there’s a apple id activation lock on it and im trying to reach/call the person who sell the phone and its not reachable , so i go to google check the imei its reported missing/stolen phone. Can you help me on this situation