One redeems maaser sheni according to the inexpensive rate: [the rate] at which the shopkeeper buys, not at which he sells; [the rate] at which the moneychanger takes perutos, not [the rate] at which the moneychanger amasses [them]. But one may not redeem maaser sheni by estimation: That which has a known value is re-deemed based on the pronouncement of one witness; but that which does not have a known value is redeemed on the pronouncement of three: such as wine that has started to sour, or fruit that has decayed, or coins that have rusted.