Shevi'is, Chapter 7, Mishnah 3. The Mishnah lists produce that carries the sanctity of Shevi'is even though it is not the main part of the fruit, and it goes on to explain the prohibition against doing business with Shevi'is produce and with several other items that carry sanctity or a prohibition.
"Klipei rimon v'hanetz shelo, klipei egozim v'haglinin - yesh lahem shevi'is v'lidmei shevi'is":
"Klipei rimon v'hanetz shelo" - the peels of the pomegranate and the netz, which is the blossom of the pomegranate.
"Klipei egozim" - the shells of certain kinds of nuts.
"V'haglinin" - the pits of certain kinds of fruit.
All of these carry the sanctity of Shevi'is: one must treat them with sanctity and use them only for eating and not for business. "V'lidmei shevi'is" - even money exchanged for them takes on the sanctity of Shevi'is.
The peels of the pomegranate and its blossom, as well as nut shells, are used for dyeing. As we mentioned earlier, items that come from fruits and produce that carry Shevi'is and that are used for dyeing also carry the sanctity of Shevi'is.
As for the pits and the stones: the Rav understands that this refers to olive pits from which oil can still be extracted, or to date pits that are fit as animal feed. Since they are fit for human consumption or for animal consumption, the law of Shevi'is sanctity applies to them. And although the Mishnah does not mention this, according to this understanding the laws of bi'ur will apply to them as well.
The prohibition of doing business with Shevi'is produce:
The Mishnah says: "Hatzaba tzovei'a l'atzmo" - a professional dyer who uses dyeing materials may use these items that serve for dyeing, but only for his own personal use. "V'lo yitzba b'sachar" - he may not use them to dye commercially and earn money, "she'ein osim s'chorah b'peiros shevi'is" - for one does not do business with Shevi'is produce.
Additional items with which one does not do business:
"V'lo bibchoros" - one does not do business with a firstborn animal. It carries sanctity and must be given to a kohen, whether it has a blemish or not. A firstborn without a blemish - the halachah is that it is brought as an offering in the Beis HaMikdash, when that is possible; and a firstborn with a blemish belongs to the kohen and is fit to be eaten, and anyone may eat from it, but financially it belongs to the kohen. And even though it has a blemish, one may not shear it or use it for work. The Mishnah teaches that as long as the animal is alive one may not do business with it, out of the concern that even with a blemished animal, someone who deals with it and spends time around it may ultimately come to violate the prohibition of shearing it or using it for work.
"V'lo bitrumos" - Terumah too carries sanctity, and there is a concern that if people trade in it, it will ultimately be eaten by those who are not kohanim.
"V'lo binveilos, v'lo bitreifos, v'lo bishkatzim, v'lo birmasim" - meat that was not slaughtered properly, which is neveilah; an animal with a treifah, a problem in its lung or the like, which renders it not kosher; and likewise creeping and crawling creatures. One may not trade in any of these items that are not kosher.
Taking field vegetables and selling them in the market:
The Mishnah returns to the subject of Shevi'is produce: "V'lo yehei loke'ach yarkos sadeh u'mocher bashuk" - one may not buy vegetables from a field during Shevi'is and sell them in the market. In fact, not only is buying them prohibited, but one may not even pick them himself and sell them in the market, since either way this constitutes doing business with them.
What is permitted is this: a person may pick them for his own needs, and if he has extra left over, his son may sell the portions he does not need, and this is not considered doing business with them. He is not required to let the produce go to waste, but may sell it. Similarly, if he took them for his own eating and not at all for the purpose of selling, he himself may sell what is left over, since these are items that remained and it is permitted to sell them.
However, regarding whatever is sold, the money exchanged for it carries the sanctity of Shevi'is, as was stated above: the money takes on the sanctity of Shevi'is once it has been exchanged for Shevi'is produce.
In summary: In this Mishnah we learned that pomegranate peels and the blossom, nut shells and pits carry the sanctity of Shevi'is, as does the money exchanged for them, because they are fit for dyeing or for human or animal consumption; that a dyer may dye for himself but not for pay, since one does not do business with Shevi'is produce; that one also does not do business with firstborn animals, Terumah, neveilos, treifos, or creeping and crawling creatures; and that one does not take field vegetables and sell them in the market, unless there is a surplus left over from picking for his own needs - and even then the money carries the sanctity of Shevi'is.