Gemara

(Mishnah): The following are forbidden three days before idolatrous festivals of idolators: to buy from or sell to them, to lend to them or borrow from them objects or money, and to pay up or collect loans;

R. Yehudah permits collecting loans from them, for this pains them.

Chachamim: Even though it pains them now, it gladdens them later.

5b - Question: Are we really concerned lest one prepares for the festival so far in advance?!

(Mishnah): On four days in the year, one who sells an animal must tell the buyer if he already sold the mother or child that same day (lest both will be slaughtered that day, and the buyer transgress), i.e. the days before Shemini Atzeres, Pesach, Shavu'os, and Rosh ha'Shanah.

Answer: One day suffices to buy an animal to be eaten. Three days are needed to prepare a Korban.

6a - Question: Is it forbidden to do business with them due to Harvachah (profit, or having more from which he can offer), or due to "v'Lifnei Iver Lo Siten Michshol" (enabling him to sin)?

Question: What difference does it make?

Answer: If the Nochri already has an animal, 'Lifnei Iver' does not apply, but Harvachah does.

6b (Reish Lakish): If one (bought or) sold to Nochrim before the festival, he may benefit from the (item or) money.

Support (Beraisa): The Isur (prohibition) to (buy and) sell pertains only to things that will last (until the festival), but not to perishables. Even things that will last, if one (bought or) sold, it is permitted.

(Rav Zvid - Beraisa): Something that will not last we may sell to Nochrim. We may not buy it (Rashi; R. Tam - accept such a gift) from them.

12b (Mishnah): There was an idolatrous festival in Beis Shan. Some stores were crowned with adornments of the idolatry (they pay taxes to the idolatry). Chachamim ruled that one may not buy from those stores.

13b: R. Yirmeyah bought bread (at a Nochri bazaar).

Rishonim

Rambam (Hilchos Avodah Zarah 9:1): Three days before the festival of idolaters, one may not buy from them, or sell to them something that lasts. One may not lend to them or borrow from them, or pay a debt to them or collect from them a loan with a document or security. One may collect from them a Milveh Al Peh, for this is like saving his money from them. One may sell something that will not last until the day of their festival, such as vegetables or a cooked food. This is in Eretz Yisrael.

Rosh (1:1): Rashi says that the Mishnah forbids buying and selling. He explains later that the concern is lest the Nochri thank his idolatry. It is forbidden due to "Lo Yishama Al Picha" - due to the Yisrael, the Nochri mentions the name of idolatry. R. Tam challenged this, for the Gemara proves that we do not forbid all buying and selling, rather, only what is appropriate to offer to idolatry. The Gemara asked why for selling animals for slaughter, we are concerned only for the day people slaughter, and here we forbid three days in advance. It answered that more time is needed for offering. However, one could say that as long as he is engaged in (preparing to) offer to idolatry, he thanks for every benefit that comes to him. He thnks that his idolatry helped him, since he was engaged in its needs. R. Tam also asked from the question whether we forbid due to Harvachah or Lifnei Iver. Rashi explained that he will than the idolatry for his profit. The question was not resolved. Also, we asked what is the difference, and answered that they argue about one who has his own animal. Why didn't it say that they argue about selling something that is not offered? If the concern is Harvachah, it is forbidden, but if the concern is Lifnei Iver, it is permitted! R. Tam explains that thanking applies only to lending and paying them, for then they profit, and borrowing from them. This is important to them, that Yisre'elim need them. They do not consider business such a profit, for the Yisrael profits like he does. Also, when he buys he is unsure if he will profit, and when he sells he worries lest he spend the money before he buys other merchandise. The Rambam forbids buying anything, for he will still have the money (on the festival). R. Tam explains that the Mishnah forbids selling to them things that they might offer. The Gemara asked if it depends on Harvachah (having extra). If so, we forbid even if he already has what to offer, for when he has more, he offers more and higher quality Korbanos and thanks his idolatry. If it depends on Lifnei Iver, it is permitted if he already has what to offer. Rav Zvid's Beraisa does not forbid buying from them things that do not last, rather, accepting them for a gift. Rashi holds that the Mishnah forbids selling to them, but we permit buying from them, for a seller is usually sad, unless he sold something that does not last. The Beraisa that supports Reish Lakish permits (selling) something that does not last. According to Rashi, it should also permit (buying) something that lasts! We can say that the Beraisa explains the Mishnah, which discusses only selling. R. Tam holds that the Mishnah forbids only selling things that can be offered, or accepting a gift. He could admit that one may not buy something that will not last. Since he longed to sell it, he rejoices when the Yisrael buys it. It is difficult to say that the Mishnah discuss selling and accepting gifts. Rather, it discusses selling and its opposite, i.e. buying.

Beis Yosef (YD 148 DH Tanya): The Beraisa is difficult for R. Tam. The Rosh said that he must explain that it discusses accepting a gift. However, this is difficult. Rather, R. Tam admits that one may not buy something that will not last. For this, he would thank his idolatry.

Ran (1b DH Dinra): We are not concerned lest a Nochri thank his idolatry for getting a loan on Ribis, for this pains him.

Poskim

Shulchan Aruch (YD 148:1): Three days before the festival of idolaters, one may not buy from them, or sell to them something that lasts. One may sell something that will not last until the day of their festival, such as vegetables or a cooked food. One may not lend to them or borrow from them objects, or lend money (Rema - without Ribis) or borrow money from them, or pay a debt to them or collect from them a loan with a document or security. One may collect from them a Milveh Al Peh, for this is like saving his money from them. When they have the upper hand, even a loan with a document is like saving from them. If it was a loan with Ribis, even if he had a security this is like saving from them.

Beis Yosef (DH Shloshah): We conclude that the three days is excluding the day of the festival.

Beis Yosef (DH v'Da): A Mishnah forbids buying from crowned stores. Rashi explains that it is not a problem of buying on the day of the festival, for a seller is sad. Tosfos explains that R. Yirmeyah bought bread because there is great demand for it, so it is like something that lasts. The Rambam holds that the Beraisa teaches that only something that lasts is totally forbidden (both buying and selling), but one may sell something that will not last. This is because nothing will remain from it for the Nochri on the day of the festival. When one buys anything, he will have the money on the day of the festival. The Ra'avad says so; the Ran says so in the name of Ge'onim, and says that this is primary.

Taz (1): Even though he is sad to sell, he will still have the money on the day of the festival.

Taz (3): The Shulchan Aruch is like Rashi and the Rambam. According to them, the Gemara could have given another difference between whether the Isur is due to Harvachah or Lifnei Iver. If it is the latter, one may sell things not offered! This is why R. Tam explained differently. Rashi and the Rambam must say that the Gemara taught another difference, that we distinguish even regarding animals that are offered. The question was whether there is concern lest he thank his idolatry for any sale, or not, like R. Tam. The question was not settled, so we are stringent.

Gra (1): All the Poskim who rule like R. Tam permit buying and selling. According to the Rosh, R. Tam is lenient (more than Rashi) only about selling. R. Tam's proof was that the Gemara did not say that matters not offered are permitted. We can say that Chachamim forbade buying and selling everything (that he does not have) due to things that are offered.

Bach (7): Also if the Nochri thanks his idolatry, the Yisrael transgresses Lifnei Iver! Rather, thanking is no worse than hugging and kissing it. A Nochri is not commanded about this, for a Yisrael is not killed for it.

Question (Taz 3): A Yisrael is killed for accepting an idolatry to be his god (and this is included in thanking it)! Rather, Lifnei Iver is only when he could not transgress without the Yisrael, similar to passing wine to a Nazir on the other side of the river. If he could transgress by himself, and the Yisrael merely helps him, this is not Lifnei Iver, like the Mordechai says. The Rema (151:1) brings an opinion that forbids. This is only mid'Rabanan. The only Torah Isur is Lo Yishama Al Picha.

Shach (3): Nowadays Nochrim has the upper hand, and they find excuses to deny loans. In any case nowadays all is permitted (Sa'if 12).