Demai, Chapter Two, Mishnah Two
The opposite of an am haaretz, one who is assumed not to have tithed his produce, is one who is neeman, which means trustworthy. He is a person that one can trust to have tithed his produce. One who buys from such a person need not treat his produce as demai. Our mishnah teaches how one can be considered neeman.
1) One who accepts upon himself to be trustworthy (neeman), must tithe whatever he eats and whatever he sells and whatever he buys, and he may not be the guest of an am haaretz.
2) Rabbi Judah says: even one who is the guest of an am haaretz can still be considered trustworthy.
3) They said to him: He is not trustworthy in respect of himself! How can he be considered trustworthy in respect of others?
Section one: For a person to be considered trustworthy, meaning other people can buy and eat his produce without having to fear that the produce was untithed, he must be scrupulous in his personal observance of the laws of tithes. He must tithe whatever he eats, sells and buys. Buying here refers to buying something in order to sell it. He also, according to the first opinion in the mishnah, may not be a guest of an am haaretz, because that would mean he is probably eating untithed produce.
Section two: Rabbi Judah argues that even one who is guest at the table of an am haaretz can still be considered trustworthy. Perhaps this person trusts that this particular am haaretz did indeed tithe his food. At the least, the person might claim that he trusts the am haaretz not to feed him (the guest) any untithed produce.
Section three: According to the sages, by eating with the am haaretz, he is not acting in a trustworthy manner in regard to that which he eats, because he may be eating untithed produce. If he is not trustworthy in regard to that which he himself eats, how can he be considered trustworthy with regard to that which he sells to others? Therefore, the sages hold that one who eats with an am haaretz is not trustworthy to sell tithed produce. Produce brought from him is considered demai.