Gemara

(Beraisa): In any place, we may buy eggs from Nochrim. We are not concerned lest they came from a Nevelah or Treifah.

Question: Why aren't we concerned lest they are from a Tamei species?

Answer (Shmuel's father): The case is, the seller said that they are from a particular Tahor species.

Question: Why must he specify the species?

Answer: If he would say only that they are Tahor, he could lie without fear of being caught.

Question: Why do we rely on his words? We should inspect the eggs!

(Beraisa): The Simanim of Tahor eggs is that one end is spherical, and the other end is pointed. If both ends are spherical, or both are pointed, they are Tamei. If the egg white is on the outside, and the yolk is on the inside, they are Tahor. If it is vice-versa, they are Tamei. If the white and yolk are mixed together, they are eggs of a Sheretz.

Answer #1: The case is, the yolk and white are beaten together in a bowl.

Objection: We may not buy such eggs from a Nochri!

(Beraisa): We may not sell eggs of a Treifah to a Nochri, unless they are beaten in a bowl. Therefore, we may not buy beaten eggs from a Nochri.

Answer #2 (R. Zeira): The Simanim to distinguish Tamei eggs from Tahor are not mid'Oraisa, so we do not rely on them.

Support: If the Simanim were mid'Oraisa, we would be able to resolve the eight doubtful species (62b) by inspecting their eggs!

Question: Since we may not rely on the Simanim, why were they taught?

Answer: If they have the Simanim of Tahor eggs, we may trust even a Nochri if he says that they are a particular Tahor species. We may not rely on the Simanim alone, for raven eggs resemble dove eggs.

Avodah Zarah 40a (Rav Berona): Fish innards and fish eggs may be bought only from an expert.

40b - Question: If there is no expert, what should one do?

Answer #1 (Rav Yehudah): If the seller says 'I salted them myself', one may rely on him (he would be afraid to lie).

Answer #2 (Rav Nachman): The seller must say 'these are the fish, and these are the innards.'

1. Rav Yehudah authorized Ada Daila to rely on a seller who says 'I salted them myself.'

Rishonim

Rambam (Hilchos Ma'achalos Asuros 3:18): If both ends of an egg are spherical, or both are pointy, or the yolk is outside and the white is inside, it is surely Tamei. If one end is spherical, and the other is pointy, and the yolk is inside and the white is outside, it could be Tamei or Tahor. Therefore, one asks the Yisrael trapper who sells it. If he names the Tahor species that it is from, we rely on him, but not if he says only that it is Tahor.

Ra'avad: This refers to a Yisrael not established to be trustworthy. If he is established not to enter a Safek, we buy from him Stam.

Magid Mishneh: This is obvious! The Rambam did not need to write this, for he already said regarding fish eggs that one may buy anything from a trustworthy seller.

Beis Yosef (DH veha'Rambam): It seems that they did not have our text, which says 'in any place, we may buy eggs from Nochrim.' Rather, it said 'in any place, we may buy eggs', like the text of the Rif and Rosh. Even so, the Magid Mishneh brings from the Ramban that even a Nochri is believed if he says that they are from a particular Tahor species. The Gemara forbids buying beaten eggs from a Nochri. This implies that one may buy whole eggs. The Tur holds that the Gemara needed to forbid in such a case, that he says from which species they are, and the buyer recognizes that this is true but perhaps they are Treifah, and a Yisrael sold to the Nochri.

Rambam (19): Therefore, one may not buy eggs from a Nochri unless he recognizes them and has Tevi'as Ayin (i.e. he knows the name of the species, and recognizes it visually - PF) that they are of Tahor species Ploni. One may never buy eggs from Nochrim if the yolk and white were beaten together in a bowl.

Beis Yosef (DH v'Al, citing Mahari ben Chaviv): The Tur says that we do not distinguish a Nochri from a suspected Yisrael. I am unsure if the Tur explains the Rambam, or argues with him. The Rambam says that we ask a Yisrael trapper. He does not discuss a trapper established to be trustworthy. Therefore, the Tur asked how a suspected Yisrael is different than a Nochri. I understand the Rambam differently, and there is no question. There is a third level of Yisrael. We never saw him sell anything that gives grounds to suspect him, but he was not established to be trustworthy. The Rambam says that we ask him. If he was surely suspected, the Rambam agrees that he is like a Nochri. Perhaps the Tur means like this, and he explains the Rambam. Logically this is sound, but his words connote that he argues.

Taz (3): The latter Perush must means that the Tur explains the Rambam, but he himself disagrees. The Tur says that also for a Nochri, it helps if we recognize (the name of the species), even if we do not have Tevi'as Ayin, and the Rambam disagrees.

Ran (21b DH b'Omer): Some say that even a Yisrael who is not established to be trustworthy must say from which species they are. The Ramban disagrees, for we say in Avodah Zarah (40a) that one is believed to say that he salted fish. Likewise, it suffices if he says that he saw that they are from a Tahor species. However, this is only when he is not suspected of feeding Isur to people. Rather, he bought from a Nochri and relies on Simanim of the innards or eggs. If he is suspected, we do not believe him.

Poskim

Shulchan Aruch (YD 86:1): If both ends of eggs are round, or (both ends are) pointy, or the yolk is outside and the white is inside, it is surely Tamei. Even if the owner says that they are from a Tahor bird, we do not rely on him.

Shach (1): He is not believed even if he is established to be trustworthy and says from which species they are and we recognize the species.

Shulchan Aruch (ibid.): If one end is spherical, and the other is pointy, and the yolk is inside and the white is outside, it could be Tamei or Tahor. Therefore, one asks the Yisrael (Rema - or Nochri) trapper who sells it. If he names the species that it is from, and we know that it is a Tahor species, we rely on him, but not if he says only that it is Tahor.

Beis Yosef (DH Simanei): When the seller said they are from a particular Tahor species, there is a Chazakah that he does not lie. He fears lest they bring eggs of that species, and we will see that they are unlike his eggs. If he would say only that they are Tahor, he could say that they are from a species that we do not recognize. The Gemara says that raven eggs resemble dove eggs. If so, why can we rely on Simanim of eggs? He could give raven eggs, and say that they are dove eggs! The Rashba answered that even though they are similar, he fears to lie, lest we compare them carefully, Surely, it is possible to distinguish eggs of different species.

Taz (2): the same applies if he named the species, but we do not recognize it. The Tur requires that it is a common species that we, i.e. most people, recognize. If it is not common, the Nochri assumes that the Yisrael does not recognize it. One who lies tries to distance himself from being caught. And even if he says that it is a common Tahor species, but does not name it, he is not believed.

Shach (2): The Shulchan Aruch rules like the Rambam, who says that we rely on a Yisrael not established to be trustworthy, but we never rely on a Nochri. Therefore, one may buy from a Nochri only if he recognizes and has Tevi'as Ayin. The Ramban Ran, Tur and many Poskim hold that we rely on a Nochri just like on a Yisrael not established to be trustworthy. I proved that this is correct from Avodah Zarah 40 and Chulin 64a.

Shulchan Aruch (ibid.): Therefore, one may not buy eggs from a Nochri unless we recognize them and have Tevi'as Ayin that these are eggs of a Tahor species. We are not concerned lest they are eggs of a Tereifah or Neveilah.

Shach (2,4): The Rema holds like I said (that a Nochri is like a Yisrael without a Chazakah), that one does not need Tevi'as Ayin. He did not comment here; he relied on what he wrote above 'or a Nochri.'

Taz (4 and Shach 5): We are not concerned lest they are from a Treifah bird, for we follow the majority.

Shulchan Aruch (2:4): If a Mumar l'Te'avon (he transgresses due to lust) has meat, and there is an expert Shochet in the city, he is believed to say that an expert slaughtered for him.

Taz (7): Why do his words help? He is not believed! Bedek ha'Bayis asked this against the Rashba. The Rashba answered that he is believed only when he names the Shochet. Then, he fears to lie. This is difficult. If so, even a Nochri should be believed. A Nochri is believed to say that eggs are from species Ploni!

Shulchan Aruch (119:19): If Reuven is suspected to eat cheese of Nochrim, and Shimon asked him to buy Kosher cheese from an expert, and Reuven says 'I bought from an expert', he is not believed. If he says 'I bought from Ploni the expert', he is believed.

Shach (37): The Beis Yosef says that in the latter case, he is believed only for an Isur mid'Rabanan. The Prishah says so even for an Isur Torah. The Shulchan Aruch is difficult, for in Siman 2, a Mumar is believed to say that an expert slaughtered for him, for a Torah Isur! (I explained that this is only if he named the expert.) I answer that there, the only concern is lest he himself slaughtered. If we know that he did not, we rely on the majority of those who slaughter, that they are experts. This does not apply here. Also, perhaps here bought from an amateur, and lies, lest he lose money. There, he need not lie. Also, there he knows that Neveilah is Asur. He would not transgress Lifnei Iver (making others transgress). Here, he thinks that Nochri cheese is permitted, so he would feed it to others. Therefore, he is believed only for an Isur mid'Rabanan. The Prishah says that he is believed about Neveilah only if he himself eats from it. This is wrong. Why does the Rema trust a Nochri to say that eggs are from species Ploni, if we recognize that it is Tahor? It is a Torah Isur! We can say that since he himself sells, we rely on him.