I bought a car from a private seller that is trash?

I bought a car from a private seller that i dont even know that only i have is a phone number, he tell me that car was running great i tried a tested just on the street it souds everytring okey, next day i went to get smog it didnto past and i started a bad noise i took it to the mecanic and they tell me that motor its broke. what can i do please help