Bit of a dilemma that one, if you report it, your not gonna get your cash back and you'll have to pay to have it repaired, if you 'threaten' to report it, get your cash back and give him the car back, hes just gonna sell it to someone else, plus you won't be able to report it then as you won't have the car. But if you don't report it they'll just carry on doing this!

Good Luck