What is the ruling on a seller telling a buyer that a piece is original and swearing to it falsely, and is he accountable for the financial and religious consequences of that?

A false oath to sell goods is a major sin. It is considered a form of deception and destroys blessing. However, it does not affect the validity of the sales contract. The sale remains valid, and its effects, such as the transfer of money to the seller and the sold item to the buyer, still apply. Even if the buyer was defrauded, they have the option to either uphold or annul the contract. For indeed, the failure to disclose a defect does not invalidate the sale; rather, it grants the buyer the option.