Final answer:
The form of listing that is illegal in many states is the net listing.
Step-by-step explanation:
The form of listing that is illegal in many states is the net listing. A net listing occurs when a seller sets a minimum amount they want to receive for their property, and anything above that amount becomes the commission for the real estate agent. This practice is considered unethical and potentially harmful to the seller, as it can create a conflict of interest for the agent. The legality of real estate practices can vary by state, so it's essential to be aware of the specific regulations in the relevant jurisdiction.