Gemara

(Beraisa): One may go to a fair of Nochrim and buy animals, slaves, houses, fields and vineyards;

He may go to their courts, for this is like saving from them.

Mo'ed Katan 10a (Rava): All Sechorah (business, e.g. buying and selling) is forbidden [on Chol ha'Mo'ed].

13a (Mishnah): We may not buy houses, slaves or animals unless they are l'Tzorech (needed for) the Mo'ed, or for the seller, if he has nothing to eat.

Tosefta (Mo'ed Katan 1:12): We may buy from Nochrim fields, houses, vineyards, slaves and animals, for this is like saving from them. He writes and enters the courts.

Rishonim

Rambam (Hilchos Yom Tov 7:22): We do not do Sechorah during the Mo'ed. We permit a Devar ha'Avud that is not always found after the Mo'ed, e.g. if ships or caravans arrived or they are about to leave, and they are selling cheaply or buying for a high price. We may buy houses, slaves or animals only l'Tzorech ha'Mo'ed.

Magid Mishnah: The Isur of Sechorah even on Yom Tov is only a decree. Therefore Chachamim did not forbid what one needs to use or resell for profit, except for houses, slaves or animals not l'Tzorech ha'Mo'ed, for such sales become known.

Nimukei Yosef (Mo'ed Katan 4b DH uv'Devar): The Ramban forbids buying from a caravan in order to resell for profit, for loss of profit is not Devar ha'Eved.

Magid Mishneh: Fields and vineyards are also forbidden; the Mishnah listed only things that are sometimes l'Tzorech ha'Mo'ed. The Mishnah permits buying houses, slaves or animals only l'Tzorech ha'Mo'ed or if the seller has nothing to eat. This refers to buying normally, but not from a caravan. I.e. other things that one needs, even if he does not need them during the Mo'ed, he may buy them, for there is no Isur Torah to buy and sell even on Yom Tov. Therefore, during the Mo'ed one may buy whatever he needs. The Rambam and Ra'avad permit in order to profit if he finds cheap merchandise. The sale of houses, slaves and animals has great publicity, so they are permitted only l'Tzorech ha'Mo'ed, or for needs of the seller. The same applies to fields and vineyards, since also they have great publicity. The Mishnah did not teach about them, for they are never l'Tzorech ha'Mo'ed.

Rambam (Hilchos Avodah Zarah 9:14): One may go to Nochri fairs and buy from them animals, Nochri slaves, houses, fields, vineyards. He writes and enters their courts, for this is like saving from them.

Kesef Mishneh: Rashi explains that one may bring documents into the Nochri courts to sign them, even though this honors them, and perhaps they will thank their idolatry. It is permitted, for this is like saving from them. This enables him to have witnesses in case there will be protests. 'This is like saving from them' refers only to bringing documents into the Nochri courts.

Rosh (Mo'ed Katan 1:23): Sechorah is forbidden because it is an exertion. Devar ha'Eved is only if one will lose from the principal. One may do Sechorah with a caravan selling cheaply or buying for a high price if there will not be a similar opportunity afterwards.

Beis Yosef (OC 539 DH v'Chosav Mahari, citing Maharik): If a periodic fair occurs on Chol ha'Mo'ed, this is like a profit that he already made. Even those who forbid Sechorah with a caravan would permit this.

Tosfos (13a DH Ein): Rabbeinu Yosef says that also other things are permitted only l'Tzorech ha'Mo'ed. One may sell Peros only l'Tzorech ha'Mo'ed. The Chidush is that even these, which are publicized, are permitted l'Tzorech ha'Mo'ed. It is not clear whether or not they must be sold covertly, like Peros, garments and Kelim.

Hagahos Ashri (Mo'ed Katan 2:18 DH v'Afilu, citing Or Zaru'a): One may buy houses from a Nochri on Shabbos. He shows to the Nochri a wallet of coins, and the Nochri signs and brings the document to the court.

Shiltei ha'Giborim (Mo'ed Katan 6b:2): We may buy from Nochrim, fields, houses, vineyards, slaves and animals, even if not l'Tzorech ha'Mo'ed, for this is like saving from them, like the Tosefta says.

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.

Shulchan Aruch (12): We may not buy houses, stones, slaves or animals unless l'Tzorech ha'Mo'ed, or if the seller or the worker who will transport them has no food.

Magen Avraham (15): One may buy anything from Nochrim, for this is like saving from them.

Mishnah Berurah (40): Some permit buying houses, fields, vineyards, slaves and animals from them, for this is like saving from them.

Bi'ur Halachah (DH Ein): The Magen Avraham and Eliyahu Rabah bring Shiltei ha'Giborim, who permits because this is like saving from them, like the Tosefta. Not all Poskim agree to this. The Tosefta concludes 'he writes and enters the Nochri courts. It says like this in Avodah Zarah 13a regarding going to Nochri fairs. Rashi explains that this refers to going to their courts. It is permitted, for it is like saving from them. The Magid (really, Kesef) Mishneh says so. Also the Gemara there discusses the reason. 'This is like saving from them' does not justify the purchase, rather, going to the courts. (However, Shiltei ha'Giborim there (Avodah Zarah 3b:2) in the name of R. Yonah connotes a little like here.) If so, also here he must explain that we permit writing and entering their courts during the Mo'ed because this is like saving from them. If so, we must find another reason to permit buying from Nochrim on Chol ha'Mo'ed. The entire law is not clear. The Gemara, all the Rishonim, Rambam and all the Poskim did not bring this Tosefta. They hold that the Halachah does not follow it. The Rambam Stam wrote the Mishnah's law, that we buy houses... only l'Tzorech ha'Mo'ed. He did not distinguish and permit buying from Nochrim in every case. This is because the Mishnah said Stam 'we buy... only l'Tzorech ha'Mo'ed.' It did not say that one may buy from Nochrim. Also, the Mishnah permits l'Tzorech ha'Mo'ed, or for the seller. This shows that there are no other Heterim.

Bi'ur Halachah: Alternatively, the Rambam and Poskim explain unlike Shiltei ha'Giborim. The Tosefta does not permit from Nochrim more than from Yisrael, rather, also from Nochrim. It discusses only Tzorchei ha'Mo'ed. There was no need to explain this, for it refers to the Mishnah. It merely permits even from Nochrim, even though one must go through their courts. One may buy, for it is for the Mo'ed. One may go through their courts, for this is like saving from them. It seems to me that the Tosefta does not discuss the Mo'ed at all! It is taught in Avodah Zarah and Eruvin regarding fairs of Nochrim. Here [in Mo'ed Katan] it lacks only the words 'one may go to a fair of Nochrim.' It was taught here, for we permit it like Devar ha'Avud, if he will not find it another day. (Also the previous Beraisa discusses Devar ha'Avud.) In Avodah Zarah, the Rosh and Ran say that the Heter regarding idolatry is because it is like Devar ha'Avud, for at the fair one is able to buy. If so, there is no Tosefta to permit, unless we will say that also here, one may go and buy on Chol ha'Mo'ed for he will not find it another day. However, if so, Shiltei ha'Giborim permits only at a fair. However, perhaps in Eretz Yisrael we permit buying houses and fields in any case, in order to settle Eretz Yisrael. In other cases, the law is not clear to me at all.

Kaf ha'Chayim (63): The Bi'ur Halachah questioned the Heter, but Mikra'ei Kodesh says that the custom is to be lenient.

Mishnah Berurah (41): One may buy a house to live in, rocks to fix a wall, and slaves to serve him.

Mishnah Berurah (42): It is permitted also if the middleman (who receives some of the money) has nothing to eat. We permit sales for the sake of the middleman or transporter only if this is the real intent of the buyer and seller.

Question (Eshel Avraham 15): We should be concerned lest one scheme [and say that he buys in order that the middleman profit]!

Answer (Kaf ha'Chayim 66): It seems that we rely on him, just like we rely on a worker who says that he has nothing to eat, even though we cannot verify his words.

Kaf ha'Chayim (65): It seems that even if the seller has money, but he will profit and spend more for Simchas Yom Tov, it is permitted, like in Sa'if 4 (regarding regular merchandise). Or, since these matters have more publicity, perhaps we permit here only if he truly has nothing to eat, like the Magen Avraham says in 542:1 [that one may hire a worker only if he truly has nothing to eat].

Rema: This is only for these matters, in which the sale is publicized.