Gemara

Some birds were Safek Nidras (clawed by a venomous animal). They were brought in front of Shmuel. He choked them and threw them in the river.

Question: Why didn't he sell them to Nochrim?

Answer: He was concerned lest the Nochrim sell them to a Yisrael.

Pesachim 40b: A ship carrying wheat capsized in a river (before Pesach). Rava permitted to sell the wheat to Nochrim.

Question (Rabah bar Livai - Beraisa): If Sha'atnez was lost in a garment, one may not sell it to a Nochri.

We are concerned lest the Nochri resell it to a Yisrael (since the Isur is not evident. Similarly, we should be concerned lest the Nochri resell the wheat to a Yisrael, since the Chimutz is not evident!)

Rava retracted and commanded to sell it to Yisraelim, a bit to each buyer, in order that it would be finished before Pesach.

Rishonim

Ran (Teshuvah 57): If some wheat cleaved (a sign of Chimutz) and it is less than one part in 60 of all the wheat, one may grind it before Pesach and keep it during Pesach, but not eat it. One may sell (the wheat) to a Nochri. We forbid only in a matter in which the Isur is not evident. Here, one can see that they cleaved and it became known in the city, so presumably it is permitted.

Question: A bear was somewhat domesticated. People used to play with it when it was chained up. It jumped on an ox. Perhaps it intended for play. Are we concerned for Drisah? May one sell it to a Nochri?

Answer (Terumas ha'Deshen 178): The Or Zaru'a forbids it, and the Rosh permits. One may not eat it, but one may sell it to a Nochri. Perhaps he will not sell it to a Yisrael, and even if he does, perhaps the Halachah follows the Rosh. The Mordechai brings that Or Zaru'a permits selling to a Nochri wheat soaked in water that did not cleave. Amora'im argue about whether it is Chametz, and Poskim argue about whom we follow. The Isur to sell to a Nochri is a decree mid'Rabanan lest he sell it to a Yisrael. For mid'Rabanan laws we follow the lenient opinion. Even though they argue primarily about a Torah law, selling it is mid'Rabanan. The Mordechai in Chulin says that if one paused during Shechitah of a bird, we are concerned lest he cut part of the Veshet. He may not sell it to a Nochri unless he kills it (without finishing the Shechitah). Usually the Kaneh is above (and he does not cut it until after Shechitah). Here also we should permit selling to a Nochri, for there are two doubts! We are stringent about such cases of a Safek Isur Torah, or a Vadai Isur without a source in Torah, e.g. bread of Yisrael that a Nochri baked. We are lenient only when Amora'im or Rabanan argue about the Halachah. Then, regarding selling we follow the lenient opinion. This requires investigation regarding many Sugyos in the Gemara.

Tosfos (Nidah 61b DH Lo): One may not sell to a Nochri a garment in which Kil'ayim was lost. Rashi says that if it were not lost, he would tear it out. Alternatively, if it were evident, he could sell the garment intact, since a Yisrael would see it and not buy it.

Poskim

Shulchan Aruch (YD 57:21): One may not sell a Drusah or a Safek Drusah to a Nochri, lest he sell it to a Yisrael.

Beis Yosef (DH Asur): We learn from the Terumas ha'Deshen.

Pischei Teshuvah (25): Shevus Yakov (1:55) permits to sell a Safek Drusah goose to Nochrim in a place where the custom is not to eat them. We are not concerned lest he sell it to a Yisrael in a place where they eat them, for this is a distant concern. He brings a proof from 86:10 (one may sell a scrambled Tereifah or Neveilah egg to a Nochri. We are not concerned lest he bake bread with it and sell the bread to a Yisrael) and 134:11 (if Yayin Nesech fell on wheat, he may bake bread with it and sell it to a Nochri in a place where Yisre'elim do not buy bread of Nochrim. We are not concerned lest Yisre'elim elsewhere buy it.)

Rema: This refers only to Treifos that are not known. One may sell a Tereifah that is known and evident to all. One may sell a Safek Tereifah if some are Machshir, even though we hold that it is forbidden. We are not concerned lest he sell it to a Yisrael, since some are Machshir.

Shach (51): The Terumas ha'Deshen himself concluded 'this requires investigation.' Also, his proof is not solid. Perhaps the Or Zaru'a holds that it is not Chametz, just we are stringent, but only about eating. This is not in our texts of the Mordechai. Therefore, one may be lenient only regarding Drisah, for there are many ways to permit it. Letter of the law, one can check it. Rashi permits whenever the place of Drisah is not externally visible. Terumas ha'Deshen discussed a domesticated bear. We do not permit selling to a Nochri a different Tereifah about which Rabanan disagree, unless there are other ways to permit it. This answers the Terumas ha'Deshen's question from the Mordechai in Chulin. This is why he did not ask from Chulin 53b and all the Poskim, who forbid selling a Safek Drusah to a Nochri, for it is a proper Sefek-Sefeka. We are lenient only when there are other ways to permit. He asked from one who paused during the Shechitah, for it is a mere stringency to forbid. The Or Zaru'a holds that it is a stringency to forbid soaked wheat. Alternatively, even if the Nochri sells it to a Yisrael, perhaps he will sell bit by bit, and it will be finished before Pesach. It seems that the Rema and other Acharonim hold like this, for they brought this law (the Heter to sell it to a Nochri) only here regarding a Drusah.

Pischei Teshuvah (26): She'alas Ya'avetz (1:72) says that the Rema permits even Treifos (e.g. due to a break) that are evident only after it is flayed and dissected. We are not concerned lest the Nochri swindle a Yisrael buyer, who assumes that it is Kosher. We do not force the seller to lose due to a possible loss of a buyer. Why should we be more concerned for the buyer? We are concerned only when Takalah can result, i.e. eating a Tereifah. He was unsure whether we are stringent in a place where there is little concern, e.g. a city that is mostly Nochrim. Surely one may not be lenient, for the Poskim did not distinguish.

Shulchan Aruch (YD 84:5): If worms were found in flour or similar things, it is forbidden.

Rema: One may not sell it to a Nochri, lest he sell it to a Yisrael.

Beis Yosef (DH v'Chasav): Hagahos Sha'arei Dura, citing Or Zaru'a, says that a case occurred in which wheat became wormy, and Rivam's father commanded to cast it in the river. He did not permit to sell it to a Nochri, lest the Nochri bake bread and sell it to a Yisrael. Also Mahariv received this from Maharish.

Bach (DH u'Mah she'Chasav she'Asur): It seems that only flour is Asur to sell to a Nochri for the reason given, lest the Nochri bake bread and sell it to a Yisrael. Regarding salt, legumes and fruits that became wormy, one may sell them to a Nochri. There is no concern lest they sell it to a Yisrael, since these often become wormy, so the buyer must check before eating, and no Takalah will result. The Ran says so. The Rema forbids selling to a Nochri something that is not a liquid, lest he sell it to a Yisrael. This is wrong. We forbid only flour, lest he bake bread and sell it to a Yisrael.

Rebuttal (Taz 9): The Bach learns from Teshuvas ha'Ran (57), who concludes 'since they are recognized, and it is known in the city, one may sell them to a Nochri.' Really, the Ran refutes the Bach! He discusses wheat that grew (after it was detached), regarding Pesach. There, all can see this, and it was known in the city, therefore he permitted selling to Nochrim. Here, it is common to buy salt, legumes or dry Peros from Nochrim without checking them at all, unless one notices a problem. Therefore, one may sell them to a Nochri only little by little, like we say about wheat that got wet (OC 467:1).

Shach (16): The Rema forbids matters in which the Isur will not be recognized, e.g. he will bake flour to make bread, or put salt into bread or other things, and sell it to a Yisrael. If the Isur is recognized, one may sell it to a Nochri, like the Rema said above (57:21). I do not understand the Bach. Surely salt is forbidden, for it can be put in fish or pickled vegetables, which Yisrael may buy from Nochrim! Also, it is not normal to check salt (Bechoros 8b). Anything that does not become wormy need not be checked. Here, worms came to the salt from elsewhere. The reason we are not concerned lest he sell the salt intact to a Yisrael is that the Yisrael would see that it is wormy.

Shach (17): One may not sell all the flour to a Nochri, but one may sell bit by bit, so that the Nochri will not sell it to a Yisrael, like the Tur and Shulchan Aruch say in OC 466:1. Isur v'Heter ha'Aruch (41:6) says that similarly, one may feed it to his Nochri slaves, even though he is obligated to feed them. However, he should not leave it a long time, lest Takalah result. Also Maharshal said so.

Shach (18): In 57:21, the Rema says that if some are Machshir a Safek Tereifah, even though we forbid, one may sell them to Nochrim, for there are two Sefekos. Perhaps the Halachah follows the opinion that permits. Even if it does not, perhaps he will not sell it to a Yisrael. The same should apply here! Here is different, for all forbid to Yisrael, like the Terumas ha'Deshen. Even though the Terumas ha'Deshen wrote in Hagahos Sha'arei Dura that Rivam permitted, and the Beis Yosef, Agudah, Roke'ach and Toras Chatas (Sof 46) bring it, their opinion is Batel to all the Poskim. Also, Hagahos Sha'arei Dura said that it is a mistake. We may rely on the lenient opinion (to make a Sefek-Sefeka) only when the Halachah is not clear. Here, all forbid, and Maharam himself forbids selling to a Nochri. (What Hagahos Sha'arei Dura cited from Or Zaru'a, Torah Chatas cites from Maharam). We permit only when those who forbid did not forbid selling it to a Nochri. Also, in Siman 57 we are lenient only regarding a Safek Drusah, because there are other reasons to be lenient.