Kanak tv center - sold me a defective exchange phone

I purchased Samsung M** phone from Kanak TV Center last week at Rs. ****.** Cash, it was exchanges devices by other customer, I have checked the phone which was working fine at that time but when I take the phone at home and download few applications to operate the phone it restarting automatically I immediately report this issue to the sell person at store but he refused to acknowledge the issue and told me that it had no issue I don't know why this exchanges phone not tested properly before selling to me and now asking me more money around ****.