Machshirin, Chapter 6, Mishnah 3. The entire masechta of Machshirin revolves around liquids and how they render food susceptible to tumah (hechsher l'kabel tumah). Our mishnah applies that principle to the marketplace: which foods on sale may we assume never came into contact with a machshir liquid, and which must we treat as already tamei because many hands, some of them possibly tamei, have handled them once they were wet.
Eggs
"Kol habeitzim b'chezkas taharah, chutz mishel mochrei mashkin." All eggs carry a presumption of taharah, except the eggs of those who deal in liquids.
The mishnah is speaking of ordinary eggs of chickens or other birds, of the sort that were brought to the shuk for sale. Regular eggs may be presumed tahor: we assume they were never wetted, so they were never made susceptible to tumah at all.
A merchant who also sells liquids, however, is a different case. His hands are constantly in contact with the liquids he is measuring out and selling, and he is not going to trouble himself to dry them off each time he moves from the liquids to the eggs. We therefore assume that his eggs became wet, which means they received hechsher l'kabel tumah. And once eggs are susceptible, the crowd of shoppers in the marketplace handling them, among whom there are surely people who are tamei, makes it necessary to treat those eggs as actually tamei.
"Umocher peiros yeveishim, tehoros." If that same liquid seller also carries dried fruit alongside his liquids and eggs, his eggs are presumed tahor. The reason is straightforward: nobody wants dried fruit to become damp. Such a merchant is careful to dry his hands every time he finishes handling liquid, so we may assume his hands were dry when he touched the eggs as well. No hechsher, therefore no concern of tumah.
Fish
"Kol hadagim b'chezkas tumah." All fish carry a presumption of tumah. Fish come out of the sea with water clinging to them, and that water gives them hechsher l'kabel tumah. Since they are then handled in the market, where some of those touching them may well be tamei, we must assume the fish themselves are tamei.
Rabbi Yehudah takes exception in three specific cases: "chatichas iltis, v'dag hamitzri haba b'kupa, v'kulyas ha'ispanin, harei eilu b'chezkas taharah." The iltis took its name from the place where it was caught and from which it reached the market, and it was sold cut into pieces. The Egyptian fish was a miniature species that arrived packed in a container, roughly what we would call sardines. The kulyas had a fine, thin skin and came mainly from Spain. All three share one trait: they stay alive for a long stretch after being pulled from the sea. Their death, which is what turns them into food, comes only long after whatever seawater clung to them has dried up, and no one would pour water over them, since dampness would spoil them. In Rabbi Yehudah's view, then, hechsher never had any opportunity to take effect, and such fish retain a presumption of taharah. The Rav points out that we do not pasken this way.
Fish brine
"Kol hatzir b'chezkas tumah." Tzir is the liquid that seeps out of fish once they have been salted. In the times of the Gemara this was a common commodity, used for dipping and for flavoring food. Being itself a liquid, no hechsher is needed for it, and since it is constantly being handled in the marketplace together with the fish by people who may be tamei, it too is presumed tamei.
The testimony of an am ha'aretz
The mishnah now turns to a further question: whose word may we accept about the status of these goods? "V'al kulam": with respect to every item listed here, the eggs, the dried produce and the brine, the ruling is that "am ha'aretz ne'eman", one who is generally not exacting in the halachos of tumah and taharah is nonetheless trusted, "lomar tehorin hein", when he reports that the merchandise is tahor, that is, that no liquid ever reached it. One category alone is left out of this leniency: "chutz mishel dagim", fish.
The logic behind trusting him is this: people who were careful about tumah and taharah regularly entrusted their goods to an am ha'aretz. They never would have done so had they not considered him reliable in this regard. That practice itself testifies that his word can be relied upon. With fish, however, we find that even those who were careful about tumah and taharah routinely handed their fish over to amei ha'aretz without concern, and that shows that in this particular area his word was never being relied upon to begin with.
Rabbi Eliezer ben Yaakov adds: "tzir tahor shenafal l'socho mayim kol shehein, tamei." Fish brine that was tahor, into which even the slightest amount of water fell, becomes tamei. The tiny quantity of water is enough to serve as a machshir, and once susceptibility has taken hold, the standing presumption of the marketplace, that people are handling it and that some of them are tamei, renders the whole batch tamei. We pasken like Rabbi Eliezer ben Yaakov: brine that had been fit becomes, through that drop of water, something we must treat as tamei in its entirety.