(b) That's wrong. It is true that in the past, the courts did not recognise that threats to detain, damage or otherwise unlawfully deal with another person's goods could constitute duress. However the courts now recognise that an illegitimate threat to goods, made to compel a person to enter a transaction, may justify setting aside the transaction.
In the present case, A consented to pay more than originally agreed only because B threatened to sell the car. This was not something B was legally entitled to do and the agreement to pay $750 for the repairs could be set aside on grounds of duress.