2nd hand car issue

[Ledonster]

I bought a car for my son from a chap selling used cars on facebook. Its been back to him within the 30 day warranty period to have an exhaust sensor changed after the engine light came on. one of the two sensors was replaced and the light came back on again a few days later so it was returned for the other exhaust sensor to be replaced. Upon return, the car has been inspected by an independent mechanic and there is no evidence of the second sensor having been replaced despite the car being kept by the seller for over a week. There is evidence of previous accident damage and repairs carried out with filler. I am not sure how to proceed. We are still within the 30 day warranty period offered by the seller. My son has called the seller who has admitted that the sensor that arrived was the wrong one so he used some catalytic converter cleaner instead and the light came off. He has offered to pay for the sensor to be replaced if the light returns but the trust has gone. I did a full background check on the car before buying it and there was no accident damage flagged.

[DE DOGS]

Buying a secondhand car is a lottery, advice is a'll;ways have the car inspected by an independent garage, obviously there is a cost involved, but it saves all the problems afterwards

[Pezza54]

You will have a stronger claim against the person that sold you the car if you can prove he is a dealer. The court regards a dealer as a trader who buys and sells cars with the intention of making a profit.
If he has more than one car advertised at any one time, the chances are he is a dealer. Check internet websites such as autotrader to see if he is advertising elsewhere.
How was the car advertised? Was it advertised accident free?