What is the ruling on the saying: "Goods sold are neither returnable nor exchangeable," with mention of all related possibilities?

The original ruling regarding a valid sales contract is that it becomes binding once the two parties separate from their place of transaction. Neither party has the right to retract thereafter, whether the seller stipulates "no returns or exchanges" or not, due to the Prophet's (peace be upon him) saying: "The two parties in a transaction have the option [to annul it] as long as they have not separated." If the buyer later discovers a defect in the sold item, he has the right to return it due to the defect, even if "no returns or exchanges" was written, as long as he was unaware of the defect at the time of purchase. He then has the choice between keeping the item or annulling the sale.