Glad you havent heard anything more from him. I have a few points:-
1. If the clutch went you wouldve noticed the slipping, my dad owned a E36 which I learned to drive in and when the clutch went it became, obvious.
2. If a Sold As Seen receipt is filled in and both parties declare they are happy with the condition of the car then nothing can be done, my Golf ownership has been 3 days of happiness and 5 weeks in the garage, but I bought it with a dodgy timing belt and worst came to worst, but I know I could not persue legal action.
3. 18 days is PLENTY of time to ruin a car. Ever watch Top Gear? Notice how many celebrities DESTROY gearboxes and clutches during the filming of one episode with the Star In A Reasonably Priced Car, just through trying to race a road car.
This guy clearly bought the car, started racing it (probably drifting it around carparks) and flooring it anytime he seen an attractive girl, and within the 18 days he has just about destroyed it. The money he would waste on a failed legal case is more than replacing the damaged components, and if he isn't COMPLETELY stupid he will know that.
Just relax, its not your fault in any way. My dads Volvo S60 T5 (250bhp petrol turbo) was sold to a relatively respectable looking family man who visited a month later looking missing tax documents because he had written it off and the DVLNI never issued him a V5 when we sent off the forms. Relax, can happen anyone.