Gemara

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!

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

Rishonim

Tosfos (Sukah 39a DH v'Leisiv): Perhaps also buying a Kli in order to profit and resell it for more is called business, and Shevi'is 7:3 forbids business with Peros Shemitah, Bechoros, Terumos, Neveilos and Treifos. The only way we find that they are the same regarding business is in this way, that the Torah explicitly permitted selling - "Oh Mechor l'Nochri."

Taz (YD 117:1): If the Isur of business is only mid'Rabanan, what is Tosfos' proof from Nevelah? There, surely the Torah permits! It is forbidden mid'Rabanan, and the Mishnah says that mid'Rabanan they are equal. I answer that Chachamim have no power to forbid what the Torah explicitly permitted. They may be stringent only where there is no explicit Isur or Heter in the Torah. This is the meaning of "Beis Din lash and punish unlike Torah law, not to transgress Divrei Torah, rather, to make a fence" (Yevamos 90b). This is obvious! No one may transgress Divrei Torah! Rather, they cannot be stringent when this transgresses [an explicitly leniency in] the Torah. Therefore, Tosfos proved from the explicit Heter, that surely Chachamim did not forbid selling. It must be permitted even mid'Rabanan, i.e. when he did not buy in order to profit.

Tosfos (Bava Metzi'a 64b DH v'Lo): Rashi learns from our Gemara that one may not lend money and live in the borrower's house, even if he deducts from the loan. The Mishnah forbids a lender to rent for less than the normal rental. This is unlike a Mashkanta (using the borrower's vineyard and deducting from the loan). There, sometimes it is worth nothing (does not produce anything), and even so he deducts from the loan. This is like a Safek of buying Peros through deducting. One constantly benefits from a house. There is no Safek. He Vadai rents for less [than its rental value]. R. Tam says that also a house is a Safek. It can fall or be burned, or he will not need it! Rather, it is permitted only when he lends on the house or field for collateral. The owner does not lose, for in any case he could not rent it to others, since it is collateral to the lender. R. Tam is difficult. Why does a lien force him to let the Peros rot? He can sell the Peros, or use them to pay the debt! Rather, a defense for R. Tam is that when the house or field is collateral, it is as if he bought it for those years [but the seller has rights to redeem it]. We say so about Mashkanta without deducting. In Erchin (31a), we call this absolute Ribis, but the Torah permitted it. R. Tam must say that it is absolute Ribis mid'Rabanan. Since the Torah explicitly permitted it, Chachamim did not want to apply to their decree there. The Mishnah says 'it is like Ribis, but it is not Ribis', i.e. even mid'Rabanan. Rava is forced to say that the Tana permits one sided Ribis (a transaction that in the end could be a sale or a loan) even mid'Rabanan.

Tosfos (Erchin 31a DH veha'Tanya): We say that Bayis Ir Chomah is absolute Ribis, and the Torah permitted it. Why is it different than Mashkanta without deducting, which is only Avak (mid'Rabanan) Ribis? Rashi says that a house is different, for one always benefits from it. R. Tam says that when the field was collateral, it is only Avak Ribis. He must say that Bayis Ir Chomah is absolute Ribis mid'Rabanan. The Tana says "but the Torah permitted it", i.e. since the Torah wrote the law of Bayis Ir Chomah, it revealed that this is not Torah Ribis.

Chasam Sofer (106): The great Chachamim overlooked Tosfos in Bava Metzi'a. R. Tam says like the Taz. However, this is only in the matter itself (a house in a walled city). They decreed to forbid Mashkanta [without deducting] in other matters [such as fields] in the same manner that the Torah (forbade - surely this is a printing mistake, and the text should say "permitted" - PF). However, Tosfos in Erchin was forced to give a weak answer (why it says "the Torah permitted it"). He did not answer like the Taz! However, Tosfos could hold like the Taz, but he holds that this is not considered explicit in the Torah, for perhaps he deducts rental from the redemption of the house. In any case, why didn't the Meforshim mention Tosfos in Bava Metzi'a?

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.

Chavos Ya'ir (15): L'Chatchilah, Chetzi Shi'ur is forbidden. However, surely one may leave the city (within the permitted 2000 Amos) on Shabbos. If one swore not to eat a k'Zayis of a loaf [or an entire loaf], we do not forbid half a k'Zayis [or half the loaf]. The same applies to matters other than food, e.g. Hotza'ah and all Melachos of Shabbos. Everything exempt on Shabbos is forbidden, with only three exceptions. I say that we do not forbid Chetzi Shi'ur when the Torah gave the Shi'ur, e.g. carrying four Amos in Reshus ha'Rabim, which we expounded (Eruvin 51a), or a Mil (2000 Amos) Techum, which we learn from the vacant area left around the cities of the Leviyim. (Another opinion says that the Torah forbids going past 12 Mil. We learn from Machaneh Yisrael in the Midbar. Chachamim saw a need to decree against going past a Mil, and found a support for this.) Otherwise, when the Torah explicitly permitted, Chachamim did not forbid due to Chetzi Shi'ur. I bring a proof from the Taz, who says that Chachamim have no authority to forbid what the Torah explicitly permits. (Even though I question the Taz below, his reasoning applies here.)

Chavos Ya'ir: We hold that Chachamim forbade benefit from Melachah on Shabbos (Kesuvos 34a), even though we expound "Kodesh Hu" - but what is done on Shabbos is not forbidden. Tosfos (39a) is difficult. What was his question? The verse discusses Tamei animals and Sheratzim. This has no connection to Nevelah, which applies only to Tahor animals! Surely this [Isur to do business with Nevelos] is only mid'Rabanan. And even if it were mid'Oraisa, this is to make a business from it, but not to sell to a Nochri when an animal in his house becomes Nevelah. Also Temei'im are permitted in such a case. We need not say that when the Torah gave a hint to permit and a hint to forbid, it authorized Chachamim to decide what is permitted and what is forbidden. Chol ha'Mo'ed is different, for there the Torah could not specify every Melachah permitted due to Devar ha'Avud. Regarding Sheratzim, Chachamim revealed that the Torah forbids when there is concern for sin.

Sha'ar ha'Melech (Hilchos Yesodei ha'Torah 5:8): Chavos Ya'ir refuted the Taz. Also, Tosfos (Sukah 40b DH Al) says that we discuss Ma'aser Sheni outside of Yerushalayim. In Yerushalayim, one may buy an animal for a Shelamim. A Mishnah in Ma'aser Sheni says so. According to the Taz, Tosfos did not need to cite a Mishnah! The verse says "v'Nasata ha'Kesef... b'Bakar..." This shows that Chachamim could forbid even what the Torah explicitly permits. On 39a, Tosfos holds like he wrote in Pesachim (23a DH Amar), that the Isur of business is mid'Oraisa.

Pischei Teshuvah (3): Tosfos in Chulin connotes unlike the Taz. Also Panim Me'iros (Zevachim 74 DH Rav) disagrees.