Correct. State law only regulates sale and carry. Ownership and purchase are not controlled by any law, state or federal.
To clarify, state law only applies to autos with blades over 2 inches long (which yes I realize is so short it includes most autos). It is illegal to carry a 2"+ auto on your person or in the passenger area of a car, and it is also illegal for any merchant to offer them for sale. Keep in mind federal law makes it illegal for merchants to sell them over state lines. And yes, if a merchant is illegally selling autos and you buy one, it's totally their problem as the seller.