How much you bought from the seller? As a buyer, you should check the phone's condition first before buying. There are several things you should do when COD. For example, turn on and off the phone, try the bluetooth, wifi (any place provided free wifi) make calls, receive the calls, check the phone keypads, battery, try the camera, turn on music player listen to the audio.. See if got faulty or not.
For tracing the phone number, you can try to file a report and go to the company of the operator see if they will help you to trace the number. But i think you may not be able to find the person who conned you..the number if using could be belong to other person.. Good luck to you.