Demai, Chapter Five, Mishnah Eight
Throughout the previous five mishnayot we learned that when a person buys from two different places he must be concerned lest one lot of produce had been tithed and one lot had not. If he tithed from one lot for all the produce he bought, he would be tithing from already-tithed produce for unithed produce, which is not permitted.
In our mishnah, the concluding mishnah in this series of mishnayot, we learn that if the purchaser is certain that neither lot had been tithed, he may tithe for one over the other, because all of the produce is known to be untithed.
1) One who buys untithed produce from two places, he may tithe from one lot for the other.
2) Although they have said one may not sell untithed produce except in the case of necessity.
Section one: As explained in the introduction, if one is certain that all of the produce that he bought is untithed, he may tithe from one lot for the other.
Section two: Here we learn that the sages forbid a person to sell untithed produce. This is because selling untithed produce might cause the buyer to eat the produce untithed. Since selling untithed produce is forbidden, we might have thought that if a seller claims that he had not tithed his produce he wouldnt be believed, because he would be admitting that he has transgressed. A person is generally not believed to say that he transgressed. Nevertheless, in this case he is believed.
The prohibition of selling untithed produce is not absolute. One is allowed to sell untithed produce in a case of necessity. The Yerushalmi explains that one is allowed to sell untithed produce to someone who had some of his tithed produce get mixed in with a large quantity of his untithed produce. In order to make this produce permitted, he needs to tithe from other untithed produce. He cant just take out tithes from that pile, because some of that pile had already been tithed. If this happens, a seller is allowed to sell to that person untithed produce to help him fix is other produce.