Under the laws of all states in this country the seller of an item is required to disclose to all potential buyers all that is known that may materially affect the desirability or price paid of an item. If the seller of the car knew that the car had frame damage before the sale to you and did not disclose the material fact to you, such is concealment a form of fraud.If you can prove concealment, your measure of damages would be the lesser of costs of repairs of diminution in value at the sale assuming you do not cancel the purchase and return the car to get your purchase money back.