Demai, Chapter Two, Mishnah 3. This mishnah deals with a person who wishes to reach an even higher level: not only to be trusted regarding tithes, but to become a chaver - one who is trusted in the laws of tumah and taharah as well, keeping them even when eating ordinary food, foods that are not Terumah.
"Hamekabel alav lihyos chaver - eino mocher le'am ha'aretz lach v'yaveish":
One who takes upon himself to be a chaver may not sell foods to an am ha'aretz, whether moist or dry, since we must assume that the am ha'aretz is in a state of tumah and will end up causing tumah - and it is forbidden to cause tumah even to ordinary food in the Land of Israel.
"V'eino lokeach mimenu lach":
He may not buy moist foods from an am ha'aretz, since moist items have already become susceptible to tumah and can easily become tamei. Dry items, on the other hand, he may buy from him: even though they would have become susceptible had they been wet at some point, the am ha'aretz is believed when he says that they never became susceptible at all, because they were never wet.
However, if the am ha'aretz says that they did become susceptible but the produce did not become tamei - he is not believed. What is the difference? Why is he believed about susceptibility but not about tumah? Because he is not expert in the details of the laws of tumah and taharah, unlike the laws of susceptibility, in which he is trusted.
"V'eino misare'ach etzel am ha'aretz":
Similar to what we learned in the previous mishnah regarding one who becomes trusted about tithes, the chaver may not be a guest in the home of an am ha'aretz, and this holds even if he has verified completely that the am ha'aretz observes the laws of tithes and that everything there has been tithed. The concern here is a concern of tumah, and that concern does not change.
"V'lo me'archo etzlo bichsuso":
Not only may the chaver not stay in the home of an am ha'aretz, he also may not host him in his own home while the am ha'aretz is wearing his own clothes. The reason: the clothes of an am ha'aretz are suspected of tumas midras - we are concerned that his wife sat on them while she was a niddah, and he is not knowledgeable in this. When the am ha'aretz enters the chaver's home in his clothes, the result is that clothes with the status of an av hatumah are being kept in the house, clothes that transmit tumah through contact and even through being carried.
It emerges that the clothes of an am ha'aretz are more severe than the am ha'aretz himself: he himself is a problem only through actual contact with his body, whereas his clothes transmit tumah even if one merely moved them. Therefore, if the am ha'aretz enters wearing clean clothes of his own, the situation can be managed; but in his ordinary clothes the problem is greater.
Rabbi Yehudah adds further categories in which one who wishes to be a chaver must be careful:
"Af lo yegadel behemah dakah" - he does not raise sheep and goats, because they tend to graze in other people's fields, and that involves a potential act of theft.
"V'lo yehei parutz binedarim usechok" - he does not make many vows and oaths, something that is viewed negatively; and likewise he does not engage in excessive levity and joking, since that too is a problem.
"V'lo yehei metamei lameisim" - he does not generally allow himself to become tamei, and he does not attend funerals and the like.
"Umeshamesh b'veis hamidrash" - he sits and learns with the Torah scholars in the beis midrash.
"Amru lo: lo ba'u eilu lichlal":
The Sages responded to Rabbi Yehudah about his list, that these matters are not directly connected to tumah and taharah, and therefore they are not considered criteria for being a chaver. They are important matters, but a person can be a chaver without them - even one who becomes tamei through contact with the dead, as long as he is careful to purify himself afterward. And if he has a need to become tamei through the dead, he is still considered a chaver.
In summary: in this mishnah we learned the requirements of one who takes upon himself to be a chaver: the prohibition against selling either moist or dry food to an am ha'aretz, out of concern for causing tumah to ordinary food in the Land of Israel; the prohibition against buying moist food from him, and the fact that an am ha'aretz is trusted in the laws of susceptibility but not in the laws of tumah; the prohibition against being his guest and against hosting him in his clothes, out of concern for tumas midras in those clothes; and finally, the dispute between Rabbi Yehudah and the Sages concerning the additional requirements, which in the view of the Sages are not part of the definition of a chaver.