How does a person who defrauded people in car sales and falsified them repent, when he does not know the individuals he defrauded?

If your friend does not know the people he cheated, nor the extent to which he cheated them, then he must take out from his money an amount that he believes will clear his conscience from their rights, and give it in charity on their behalf. This is done by estimating the value of the defects he concealed or what he seized through cheating. Ibn Taymiyyah said: "If one owes a right but does not know the identity of the rightful owner, then he must do what will clear his conscience."