No there's nothing illegal about selling a car or any other vehicle with a rusted frame or any rust on the car or vehicle.
Just as long as you disclose the condition of the car to the buyer and sell the car or vehicle as is then you're doing nothing illegal when selling the car or vehicle when it has a rusted frame.
I've sold cars that had rusted frames and bodies underneath them and the buyer bought the vehicle as is and nothing was or is illegal about that.
Many cars are sold daily with rusted frames and just because the car has a rusted frame doesn't mean it's a bad car and unsafe to drive.
I've driven cars and trucks with rusted frames and nothing bad has happened.
Even my truck right now has some rust on the frame and if I wanted to sell the truck I could do so.