Demai, Chapter Five, Mishnah Six
1) One who buys from a wholesaler, and then buys from him again, he may not give tithes from the one [purchase] for the other, even when both came from the same basket and even from the same kind.
2) But the wholesale merchant may be trusted if he says that both came from the same one.
Section one: This mishnah discusses buying produce from a wholesaler, a person who buys from many different farmers and sells to many storeowners. If he buys once from this wholesaler and then buys from him again, he must tithe separately for each purchase, lest the wholesaler bought from two different people, one of whom already tithed the produce and one who did not. This rule holds true even if he buys the second purchase from the same large basket and of the same type.
Section two: The wholesale merchant may be trusted if he says that both purchases came from the same seller. Note that he is not saying that this person tithed. The merchant cannot automatically be trusted to say that someone tithed. He may be trusted only to say that he bought from the same person, so that the one buying from him can now tithe from one purchase for the other.