Is it Islamically permissible to purchase cars with insurance payments at a low price, then ship them abroad, given the resulting pursuit of their owner? And is taking a commission as a broker for this operation considered a sin?

It is not permissible to buy a car or act as an intermediary in its sale as long as it is encumbered by the rights of banks or others, and permission for its sale has not been given, because of the Almighty's saying: "And do not cooperate in sin and aggression." Selling it for less than its value is evidence of deceit, and it is not permissible to assist a deceiver. The paths to lawful earnings are many, and in the Hadith (definition): "Indeed, no soul will die until it completes its term and receives its full provision. So fear Allah and be moderate in seeking [provision]."