A new bill has been introduced into the US House of Representatives that attempts to make ESRB ratings legally binding."ratings labels (appear) on video games and to prohibit the sales and rentals of adult-rated video games to minors."
If passed, any person who sells an adult game to a minor could be in line for a fine "of not more than $5,000 per violation."
In the current situation, there is no legal precedent to ensure shops stick to the ESRB ratings. Although most shops abide by the 'rules', many are also happy to overlook the guidelines for a sale.