Gemara

A moth fell into R. Chiya's flax (which was soaking in water). He wanted to chase it out with blood. Rebbi told him to slaughter a bird over the water.

Question (Beraisa): If one slaughters only for the sake of the blood, he must cover the blood. The only solution is to do Nechirah or Akirah (instead of Shechitah.)

Version #1 (Rav Dimi): Rebbi told him to make the bird a Treifah before slaughtering it.

Version #2 (Rav Dimi): Rebbi told him to do Nechirah.

Shevi'is 7:3 (Mishnah): One may not do business with Peros Shemitah, Bechoros, Terumah, Neveilos, Treifos, Sheratzim u'Rmasim.

7:4 (Mishnah): If a hunter of Chayos, birds or fish happened to catch Tamei species, he may sell them;

R. Yehudah says, even one who chanced upon it may be Loke'ach (take or buy) and sell;

Chachamim forbid.

Pesachim 23a - Question: Regarding Sheratzim it says "Sheketz Hu Lo Ye'achel," yet one may benefit from them!

(Mishnah): If hunters of Chayos, birds and fish happened to catch Tamei species, they may sell them to Nochrim.

Answer: There is different, for it says "Lachem". They are yours (to benefit from them).

Question: If so, it should be permitted to hunt them l'Chatchilah (the Mishnah permits only if one happened to catch them)!

Answer: It says "Yihyu" - they retain their initial status.

Bava Kama 80a (Beraisa): If a convert inherited dogs and pigs, he need not sell them all immediately. He may sell them little by little.

Rishonim

Rambam (Hilchos Ma'achalos Asuros 8:16): Any thing that one may not eat but one may benefit from it, one may not do business with it or intend to do his Melachah with it, except for Chelev, for it says "it may be used for any Melachah." Therefore, one may not do business with Neveilos, Treifos, Sheratzim u'Rmasim.

Rambam (17): If a hunter chanced upon Tamei species and trapped them, or he caught Temei'im and Tehorim (in his trap), he may sell them. He may not intend to work with Temei'im. One may do business with milk that Nochrim milked not in front of a Yisrael, or their cheese, and similar matters.

Rambam (18): The general rule is, one may not do business with anything that the Torah forbids. One may do business with anything forbidden mid'Rabanan, whether it is a Safek or Vadai.

Poskim

Shulchan Aruch (YD 117:1): Anything that the Torah forbids, even if one may benefit from it, if it is special for food, one may not do business with it (Rema - or accept it for collateral), even to buy it to feed to one's Nochri workers.

R. Akiva Eiger: This applies even to Safek Isur.

R. Akiva Eiger: It is not clear whether cooking Isur for Nochrim and being paid for his toil is considered business.

Gra (3): The Terumas ha'Deshen was unsure about accepting it for collateral.

Shulchan Aruch (ibid.): The only exception is Chelev. If a hunter happened to catch Tamei Chayos, birds and fish (Rema - or one happened to get a Neveilah or Tereifah in his house), he may sell them, as long as he does not intend for this.

Beis Yosef (DH u'Mah she'Chasav Aval): The Tur is from the Rambam. He permits when a hunter set a trap and caught only Temei'im, or when he caught Temei'im and Tehorim. He did not need to teach the latter; it follows from a Kal va'Chomer. Alternatively, had he taught only when he caught only Temei'im, one might have thought that Chachamim permitted only then, lest all his toil be for naught, but when he caught both, he may not sell the Temei'im. Had he taught only when he caught Tehorim and Temei'im, one might have thought that Chachamim forbid when he caught only Temei'im, for the trap was (useful) only for them. Orchos Chayim permits selling only Tehorim and Temei'im together, but not individually. Since he is a hunter, people might say that he intended for the Temei'im. Therefore, he must sell it immediately. He cannot wait to fatten it. What is his source to forbid selling the Tamei by itself? The Rambam and Tur permit. Perhaps he explains the Mishnah in Shevi'is like R. Shimshon. R. Yehudah permits even one who is not a hunter. The Halachah follows Chachamim. The Yerushalmi explains that they permit only hunters, for the government taxes them. (Since nowadays there is no tax, even hunters may not sell Temei'im alone.) I say that since the Poskim did not distinguish, it is permitted even nowadays.

Shach (9): Orchos Chayim is correct. One must sell them together. This is unlike one who trapped only Temei'im. If we would not permit him to sell just Temei'im, he will lose all his toil!

Taz (3): The Beis Yosef says that the Halachah follows Chachamim. It is permitted only if his profession is hunting.

Taz (4): Craftsmen buy skins of big goats only if they were killed through Nechirah, but not if they were slaughtered. One may do Nechirah. It is not Bal Tashchis, like I explained (116:1,6). This is why one may give exposed water to a Tamei animal, because he benefits from this. Semag (Lavin 229) says that Bal Tashchis does not apply whenever the benefit exceeds the destruction. In Chulin, we permit to do Nechirah or Ikur to use the blood to dye wool. This is not like doing business with Neveilos, since he used a permitted animal. It is like one who happened to have a Neveilah among his animals. Since he begins with Heter, he may finish (with Isur). However, R Binyamin says that he must discard the meat, for we do not do business with Neveilos. This is wrong. If so, the Gemara should have said that he throws the meat to dogs! Also, if so we should forbid Nechirah, lest he not discard the meat, and come to eat it! The Rashba says that this is why one may not engage in Neveilos and Treifos. Rather, if one deals with Heter, e.g. for the sake of the blood or skin, later he may sell the meat to Nochrim, like one who happened to get a Neveilah. If one does not know the laws of Shechitah and wants to sell the meat to a Nochri and slaughter b'Isur, surely this is forbidden. He makes a Neveilah in order to sell Neveilah.

Shach (5): Tosfos (Pesachim 23a DH Amar) says that if one inherited Tamei species, this is like what one chanced upon (and he may sell it). The Tur and Shulchan Aruch (CM 409:4) connote similarly.

Shach (6): Teshuvas Menachem Azaryah says that even a casual hunter, or any person who chanced upon Tamei animals, may sell them. However, even on occasion one may not buy them to resell them. If one transgressed and bought, Chachamim did not fine him to forbid Hana'ah.

Bach (2): The Tur teaches that we permit not only a hunter lest his toil be for naught, and also because he must pay taxes. Rather, even one who has Tahor animals, and a Neveilah occurred, he may sell it. It seems that only he may sell it, to minimize his loss. Another Yisrael may not buy it from him to sell to a Nochri for a profit. This is business!

Shach (8): It seems that Tosfos (Sukah 39a DH v'Leisev) and R. Shimshon (Shevi'is 7:3) agree. Masas Binyamin (25) forbids Nechirah of goats in order to sell their skins, and so says the Bach (OC 551:10).

R. Akiva Eiger: Shai la'Mora says that if one received an Isur for a gift from a Nochri, he may sell it. It is like something he chanced upon.

Beis Yosef (DH v'Chen): The Torah explicitly permits selling a Neveilah to a Nochri.

Rema: He must sell it immediately. He cannot wait to fatten it.

Gra (7): We infer this from Bava Kama 80a (which permits selling slowly only regarding inheritance).

R. Akiva Eiger: One may not keep it to make cooked foods from it.

Rema: Similarly, one may collect a loan from Nochrim from Tamei species, for this is like saving from them (if not, perhaps he would never collect at all).

Gra (8): This is why Chachamim permitted collecting from them on the day of their festival.

Shach (11): One may not keep them to profit more than the debt he was owed, but he may keep them until he can sell them for the debt, even if they are pigs.

Shach (12): Orchos Chayim permits only in a case that is like saving from them. This is wrong. Stam is considered saving from them. R. Yerucham says that collecting a debt is like inheritance. One may sell them slowly.

Rema: One may do business with anything forbidden only mid'Rabanan.