Demai, Chapter Six, Mishnah Six
1) Bet Shammai says: a man may sell his olives only to a chaver (an associate).
2) But Bet Hillel says: [one may sell them] even to one who only] tithes.
3) And the pious among Bet Hillel used to act in accordance with the words of Bet Shammai.
Section one: Bet Shammai forbids one to sell olives to anyone but a chaver, a term which refers to a person who not only tithes, but also observes a high level of purity, eating all of his food while in a state of purity. The problem is that the person who isnt cautious about the laws of purity will press the olives while impure, thereby causing the terumah which has not yet been separated from the olives to become impure. Since it is forbidden to cause terumah to become impure, a person may not sell his produce to a non-chaver.
Section two: Bet Hillel agrees that it is forbidden to sell produce to one who doesnt tithe, but we dont have to know that the person is fully observant of purity in order to sell them olives. There is a possibility that the purchaser may eat the olives before he presses them, and the olives dont become receptive to impurity until their oil starts to ooze (food cant become impure until it comes into contact with one of seven liquids). Since it is not certain that the impure purchaser will end up pressing his olives and then make them impure, the seller can sell them to him. However, he must know that he is going to tithe them.
Section three: Interestingly, the mishnah concludes by noting that certain more pious members of the Bet Hillel school acted like Bet Shammai. They seem to have believed that the other members of their school were being overly lenient on this issue, since it is almost certain that one who buys olives does so to make olive oil. Therefore, they, like the members of Bet Shammai, refrained from selling olives to those who they knew would make them impure.