What is the ruling on selling a defective car by saying, "I have sold you the car as mere iron," with the buyer's knowledge of that, and is it obligatory to disclose its defects to him if he asks about them, despite the aforementioned agreement?

If you become aware of a defect in the sold item, you are obligated to disclose it to the buyer. It is not sufficient merely to inform him that you sold him iron and for him to agree to purchase it, because concealing a defect is deception and misrepresentation. If you conceal the defect, the buyer has the right to return the item. However, if you were unaware of the defect and stipulated to the buyer that he absolves you of any defect he finds, then this is permissible.