Both the buyer and the seller are governed by the [law of] fraud. Just as [the law of] fraud [applies] to a layman, so does [the law of] fraud [apply] to a merchant. R’ Yehudah says: [The law of] fraud does not [apply] to a merchant.  The one who has been imposed upon has the upper hand. If he wishes, he may say to him: “Give me my money”, or: ”Give me what you defrauded me.”