Demai, Chapter 4, Mishnah 6. The Mishnah deals with a person who has arrived in a new city and knows no one there. He has no acquaintance with the reliable people there, and he does not know whom to rely on in matters of tithes and which of the residents is trustworthy in them.
The ruling in the Mishnah:
One who enters a city and knows no one there, and asks whomever he meets: "Mi kan ne'eman? Mi kan me'aser?" - "Who here is trustworthy? Who here tithes?" - in the answer we must distinguish between two cases:
"Ani" - "I am." If the one asked says of himself that he is the trustworthy one, he is not believed on the basis of his own statement alone, since we have no knowledge that he has an established status of trustworthiness.
"Peloni ne'eman" - "So-and-so is trustworthy." If he pointed to another person, that person is trustworthy.
The reason for the leniency:
The Rambam writes that there is a principle that a person does not sin where he stands to gain nothing from it. In this case the one asked has nothing to gain, and therefore he would not send the questioner to a person who is not genuinely reliable, and for that reason the questioner may rely on him.
The parameters of the leniency:
This leniency was stated specifically regarding someone who is new in the city, and it was in his case that the Sages were lenient, as the Yerushalmi says: because he needs to eat and has no other option. For that reason they were lenient to allow him to rely on such a statement, together with the principle that a person does not sin when he himself gains nothing from it, and therefore he is believed when he says that a certain person is trustworthy.
Furthermore: if he went to that person to whom he was sent, and asked him "Mi kan mocher yashan?" - "Who here sells old produce?" - and the man answered that specifically the very person who sent him there is the one who sells it - even though it appears that the two are doing each other a favor, one testifying about his fellow that he is trustworthy and the second testifying about him that he sells old produce, as though they had made a joint arrangement - nevertheless they are believed. The Sages were lenient with a visitor in the city, allowing him to rely on them temporarily, until he settles there and clarifies precisely whom he can rely on.
There are two conditions for this leniency:
Thirty days: The leniency applies only as long as he has not stayed in the city for thirty days. Once thirty days have passed, he must buy from someone who has become established in his own eyes as a trustworthy person, and this is so even if he still knows no one there.
That he knows no one: If he does know someone in the city, then he is obligated to investigate, and he may rely on such a statement only when he knows no one in the city at all.
"Mi kan mocher yashan?" - two explanations:
Some understand it simply, that old wheat is generally the finer wheat, and therefore pointing to a person as the one who sells the old produce is a compliment and a benefit to him. According to this, it appears that the two are benefiting one another, and there is room for a concern of mutual favors and reciprocity, each one seeking to benefit his fellow, and the Mishnah teaches that there is no need to be concerned about this.
Others wish to understand that the discussion concerns the prohibition of chadash, that is, before the sixteenth of Nisan, before the Omer was brought, until which time there is no permission to eat from the new grain. According to this understanding, the Mishnah teaches that the amei ha'aretz were suspected not only of failing to separate tithes, but even of the prohibition of chadash, and for that reason it was relevant to him to know who here sells old wheat.
Other commentators reject this understanding, because nowhere do we find a concern that an am ha'aretz would sell produce subject to the prohibition of chadash, or that he would sell something upon which the prohibition of chadash rests. They therefore reject this interpretation and explain the matter simply, that the reference is to the finer wheat, which is finer by virtue of being old.
In summary: In this Mishnah we learned that one who enters a city and knows no one there does not rely on someone who says of himself "I am trustworthy," but does rely on someone who testifies about another that he is trustworthy, for the reason that a person does not sin where he stands to gain nothing, and because he needs to eat and has no other option. We saw that we are not concerned about mutual favors when each of the two testifies in his fellow's favor, and we examined the conditions of the leniency - thirty days and that he knows no one in the city - as well as the two explanations regarding the sale of old produce: the superior quality of the wheat or the prohibition of chadash.