Shevi'is Chapter 7, Mishnah 4. In the previous mishnah we learned that one may not conduct business with a firstborn animal, and likewise that it is forbidden to trade in non-kosher animals. Our mishnah comes to add to these two laws and to qualify them.
Selling a firstborn that was bought for a meal:
"Lakach bechor l'mishteh b'no o la'regel" - a person who bought from a kohen a firstborn animal that has a blemish, and is therefore permitted to be eaten even by someone who is not a kohen, and he bought it in order to eat it at his son's wedding or on Yom Tov. "V'lo tzarchu" - and in the end it turned out that he had no need for it, since another animal was obtained. In such a case - "mutar l'michro", and he need not lose his money. The Tosefta adds, however, that he may not sell it for more than he paid for it, that is, he may not profit from it, but the sale itself is permitted.
Non-kosher species that come the way of trappers:
"Tzadei chayah, ofos v'dagim shenizdamnu lahem minei tme'im - mutar l'michram" - those whose profession is trapping wild animals, birds and fish, and non-kosher species happened to come into their hands, may sell them immediately after catching them. This is not considered conducting business, since that was not their intention; rather, they came upon them in the course of their work.
Rabbi Yehudah says: "Af mi shenizdamen lo l'fi darko - lokeach u'mocher, u'vilvad shelo t'hei umanuso b'kach" - even an ordinary person, who is not a professional trapper, who happened upon non-kosher species, may buy them and sell them in a non-professional manner, provided that this does not become his regular business, but rather that he earns his living from something else.
"V'chachamim osrin": one who is not a professional may not purchase them and sell them, even if they came his way incidentally and even though this is not his business.
The Rosh explains the reason for the distinction: with professionals the Sages were more lenient, since they were obligated to pay a certain tax to the king and the government, and therefore they permitted them to sell those non-kosher species that came their way, in order to ease the payment of the tax for them. Other people, who do not have this difficulty, were not granted this permission. In any event, even the Sages agree that trappers of wild animals and birds are permitted to do this.
What is the difference between the Sages at the end of the mishnah and the first opinion?
Both the first opinion and the Sages permit a professional to sell non-kosher species that came his way, and forbid it to someone who is not a professional. If so, what is the difference between them?
Some explain that according to the first opinion, even a professional was not permitted to go out and search for those animals, and it was permitted to him only when they came into his hands by chance - as the wording indicates: "that came their way," meaning that it happened on its own. According to the Sages at the end of the mishnah, however, a professional may even go out and search for them, so that he will be able to pay that tax.
In summary: in this mishnah we learned two permissions alongside the prohibition against conducting business: a firstborn that was bought for his son's wedding feast or for Yom Tov and was not needed may be sold, provided that he does not profit from it; and non-kosher species that come the way of trappers of wild animals, birds and fish may be sold, since this was not their original intention. On this Rabbi Yehudah and the Sages disagree: Rabbi Yehudah permits it even to an ordinary person who happened upon them in the course of his way, while the Sages forbid this and permit it to professionals alone because of the tax obligation, and they even extend the permission for them to go out and search for those animals.