Demai, Chapter Four, Mishnah Six
This mishnah deals with a scenario where two people offer compliments as to each others produce. The question is: do we fear that they are colluding and therefore not trust them?
1) One enters a city and doesnt know anyone. He says: Who here is trustworthy? Who gives tithes here? One person one responds: I am. He may not be trusted.
2) [But if] he replied: So-and-so is trustworthy, he may be trusted.
3) He went to buy from so-and-so, and he asked him: Who here sells aged produce? and so-and-so replied: He sent you to me, though they appear to be repaying each other, they may be trusted.
Section one: This person is not trusted because a person cannot testify about themselves that they tithe. Only others may testify concerning them.
Section two: Although the person is not believed to say that he himself tithes, he is believed to say that someone else tithes. This is, after all, the general way that a person would find out if someone titheshe would ask around about him.
Section three: The person who has now bought produce from someone who is trustworthy to have tithed now asks that same person, who in the town sells aged produce, which is better than new produce. He sends the buyer back to the original person, the same one that was not believed in section one. It looks as if they are scratching each others back. They both know that a person is not believed to compliment his own wares so they compliment each others. Despite this, the mishnah rules that each is believed. Albeck explains that the visitor to the city can treat this case leniently because as a visitor it will be hard for him to determine who is trustworthy.