Gemara

(Mishnah): Sellers of produce, garments and Kelim may sell covertly for the need of the Mo'ed;

R. Yosi says, they chose to be stringent.

Question: Were they stringent not to sell at all, or did they choose to be stringent to sell covertly?

Answer (Beraisa): Sellers of produce, garments and Kelim may sell covertly for the need of the Mo'ed;

R. Yosi says, merchants of Tiverya were stringent not to sell at all.

Rav Huna permitted spice vendors to sell normally in the market.

Question (Rav Kahana - Beraisa): (On Chol ha'Mo'ed) a store open to a Stav may open and close normally. A store open to Reshus ha'Rabim opens one door and locks the other. On the last day of Chol ha'Mo'ed we fill the markets of the city with produce, to honor the coming Yom Tov.

Inference: If not to honor the coming Yom Tov, it would be forbidden!

Answer: We are stringent about produce if not to honor the coming Yom Tov, but not about spices.

Rishonim

The Rif and Rosh (2:19) bring these teachings (except for the discussion of R. Yosi's teaching).

Nimukei Yosef (DH Gemara): The need to close one door is for Tzeni'us. The Stav is surrounded by three walls and has a roof over it, so this is enough Tzeni'us. We are stringent about produce lest people suspect that the buyer is stocking up for the year. This does not apply to spices, for most people buy them for immediately consumption.

Rambam (23): Sellers of produce, garments and Kelim may sell covertly for the need of the Mo'ed. A store open to a corner or a Mavoy may open normally. If it is open to Reshus ha'Rabim, it opens one door and locks the other.

Magid Mishnah: Because they sell the entire year, they must sell covertly.

Rambam (ibid.): On the last day of Chol ha'Mo'ed Sukos we fill the markets of the city with produce, to honor the coming Yom Tov. Spice vendors may sell normally in the open.

Magid Mishnah: We honor the last Yom Tov of Sukos because it is an independent festival, but not the last Yom Tov of Pesach. Spice vendors may sell normally in the open.

Poskim

Shulchan Aruch (OC 539:1): All Sechorah is forbidden, both buying and selling.

Rema: If one has an opportunity for a big profit during the Mo'ed he may sell covertly and spend more for Simchas Yom Tov than he originally planned.

Beis Yosef (DH Kol): The Rosh forbids Sechorah due to exertion. It seems that he and the Rambam forbid even covertly. However, the Roke'ach and Agur permit covertly. The Yere'im permit in the house, for it is not much exertion. One may rely on them to sell covertly if he will profit greatly and spend more for Simchas Yom Tov than he originally planned.

Mishnah Berurah (3): It is similarly permitted to buy covertly. This distinction refers to Sechorah, but Melachah is forbidden whether it is covert or open.

Kaf ha'Chayim (6,13): One without a store may sell Kelim openly (to get money to spend for the Mo'ed). Likewise, commerce may be done openly if it is clearly a rare opportunity, e.g. caravans (Sa'if 5).

Shulchan Aruch (10): Sellers of spices or vegetables or anything that does not last may sell normally in the open, for all know that it is for the need of the Mo'ed.

Mishnah Berurah (33): The same applies to sellers of fragrances. Even though they last, all know that it is for the need of the Mo'ed. Therefore, even if a Nochri comes to buy one may sell to him.

Shulchan Aruch (11): Sellers of produce, garments and Kelim may sell covertly for the need of the Mo'ed. A store open to a corner or a Mavoy may open normally.

Mishnah Berurah (34): Since these last until after the Mo'ed, people will suspect that one buys for after the Mo'ed, therefore they must be sold covertly. One who does not sell them the entire year may sell openly on Chol ha'Mo'ed.

Mishnah Berurah (35): This discusses a Mavoy open on only one end. If it is open on both ends, it is like Reshus ha'Rabim.

Shulchan Aruch (ibid.): If it is open to Reshus ha'Rabim, it opens one door and locks the other. On the last day of Chol ha'Mo'ed Sukos we fill the markets of the city with produce, to honor the coming Yom Tov.

Kesef Mishnah: We do not adorn the markets before the last Yom Tov of Pesach, but the Heter to sell openly applies.

Kaf ha'Chayim (60): The reason not do so on Pesach is because most people buy before Pesach, to check if Chametz grains are mixed in. The custom is to adorn even on Pesach.

Rema: One may buy on Chol ha'Mo'ed for Yom Tov Sheni.

Beis Yosef (DH Yesh Omrim): Our text of the Tur says that the Rosh was careful not do so. Other texts say that the Rosh was not careful about this. This is the correct text, for this is the custom.

Magen Avraham (14): We do not say that since the calendar is fixed nowadays, Yom Tov Sheni is really Chol and one may not prepare for it on Chol ha'Mo'ed.

Gra (DH u'Mutar): We bless on Yom Tov Sheni (Kidush, Shehecheyanu,...) lest people disgrace it, even though this is a Safek Berachah l'Vatalah (Shabbos 23a). All the more so we may buy on Chol ha'Mo'ed for Yom Tov Sheni!

Kaf ha'Chayim (62): The Eshel Avraham was unsure if the opinion that forbids Meleches Chol ha'Mo'ed mid'Oraisa would permit proper Melachos for Yom Tov Sheni. R. Akiva Eiger says that we do. However, a Ben (resident of) Eretz Yisrael may not do Melachah for the needs of Yom Tov Sheni for a Ben Chutz la'Aretz.