Demai, Chapter 4, Mishnah 5. In the previous mishnayos we learned that on Shabbos the Sages were lenient in matters of demai and allowed one to rely on an am ha'aretz. This mishnah lists cases in which he may be relied upon even on weekdays.
Sending someone who is not trusted to buy from someone who is trusted:
A person says to someone who is not trusted regarding tithes: "Kach li mimi shehu ne'eman umimi shehu me'aser" - "Buy for me from someone who is trustworthy and who tithes"; that is, I do not want you to bring from someone like yourself, but from one who is trustworthy and regularly separates tithes. The mishnah rules: "Eino ne'eman" - he is not trusted. The reason: the agent may buy from someone who is not at all worthy of trust, and then justify himself after the fact: "In my eyes he is trustworthy, and I believe him" - for as he defines it, trustworthiness is a subjective matter, left to his own judgment.
"Me'ish peloni - harei zeh ne'eman" - "From so-and-so - he is trusted":
But if the sender specified a particular person whom he knows to be trustworthy, and instructed him to bring specifically from that person, then he is believed when he says that he did indeed receive it from that person. Why is he believed here? Because the matter can be verified: one can approach that person and ask whether the agent really came to him and bought from him, and the agent is afraid of being caught in a lie. Here he does not have the excuse available to him in the first case, since this is not a subjective determination but an objective fact: either he received it from that person or he did not - and therefore he is afraid to lie.
"Lo metzativ velakachti lecha me'acher shehu ne'eman" - "I did not find him, so I bought for you from someone else who is trustworthy":
And what is the ruling if he went to buy from that particular person, and came back and said: I did not find him, so I bought for you from a different person who is trustworthy? He is still not believed, because he is not a reliable authority to judge who is trustworthy and who is not, and one may not rely on a person who is not trusted regarding tithes for such a determination.
In summary: This mishnah teaches that one who is not trusted regarding tithes is not believed when he is sent to bring "from someone who is trustworthy," since the determination is left to his own subjective judgment; but when he is sent to a specific "so-and-so" he is believed, because his words can be verified and he is afraid of being caught in a lie. And if he came back and said that he did not find that person and bought instead from someone else whom he considers trustworthy, he is once again not believed, just as in the first case.