משנה על בבא קמא 7:3

If one buys a first-born animal for his son’s [wedding] feast, or for a festival, and then decides that he has no need for it, he may sell it. Trappers of wild animals, birds and fishes, who chanced upon unclean species, may sell them. Rabbi Judah says: also a man who happened to chance one upon by accident may buy or sell, provided that he does not make this into his regular profession. But the sages prohibit.