Avodah Zarah, Chapter 1, Mishnah 5. In the four mishnayos we have just completed, we learned how one must conduct himself with idol worshippers around their festivals and holidays. From here onward the Mishnah turns to the question of functioning within the economy and society of idol worshippers throughout the entire year.
"Eilu devarim asur limkor l'oved kochavim":
The Mishnah lists items that may not be sold to an idol worshipper at any time. The list is based on what was common in the time of the Mishnah in Eretz Yisrael, and it is understood that it must be adapted to each and every society in which a person lives in later generations.
"Itztroblin uvenos shuach uvfetotroseihen" - pine cones and choice varieties of figs while their stems are still attached to them. It was a widespread custom to tie up and hang fruits of this sort - pine cones, nuts, figs and the like - before the idol as a kind of offering. Therefore, when an idol worshipper asks for fruits of this sort, the assumption is that this is what he intends them for.
"Ulevonah" - frankincense is a type of incense: a dried, hardened resin that comes from the boswellia tree, and it is burned as incense. The assumption is that an idol worshipper in the time of the Mishnah in Eretz Yisrael has no need whatsoever for frankincense for any purpose other than idol worship.
"Vetarnegol halavan" - white roosters are only a certain portion of the overall rooster population, but they were a preferred offering for idol worship. Therefore, one who asks for a white rooster is presumed to intend to offer it to the idol.
The dispute between the Tanna Kamma and Rabbi Yehudah:
We learned: "Rabbi Yehudah omer: mutar limkor lo tarnegol lavan bein hatarnegolin". First we must clarify the points on which the Tanna Kamma and Rabbi Yehudah agree:
If a person comes and asks for roosters in general, and one of them happens to be white - it is permitted to sell them to him, since one who asks for a rooster without specification is asking for it for his business needs and not for idol worship.
If he comes and asks specifically for a white rooster - it is forbidden to sell it to him, and even Rabbi Yehudah agrees to this. The very fact that he is particular about its color shows that he wants it for a specific purpose, and not for nothing.
The question is what the law is when an idol worshipper asks a Jew for a white rooster, a black rooster and a red rooster. Do we say that he designates the white one for idol worship and the black and red ones for other needs, or perhaps his very request for a variety of roosters of different colors proves that there is no intent of idol worship here? This is the subject of the dispute: the Tanna Kamma holds that it is permitted to sell to him, and such is also the opinion of Rabbi Yehudah - one who asks for a white rooster among a variety of colors may be supplied with it.
The permit to sell by inflicting a blemish:
Even regarding the sale of a single white rooster to an idol worshipper there is a permit, namely, by cutting off one of the rooster's toes. In this way the rooster becomes blemished in a manner such that no self-respecting idol worshipper would offer it as a sacrifice. Even if he is not particular about other types of blemishes that Jews are particular about in their offerings, about a missing toe he will be particular, and therefore it is permitted to sell him the rooster after removing the toe.
It is worth noting that this is an example of a case in which the Torah forbids causing pain to animals, but where there is a financial loss to a Jew there is room to be lenient. The level of suffering caused to the rooster in this case is not of the sort that technically forbids the Jew from selling it, provided that the sale is done in the permitted manner.
All other items:
In all other cases - general items that can serve a variety of purposes - the ruling depends on what the buyer asks for. Consider a Jew who runs a hardware store, and an idol worshipper comes in and asks for a chisel: must we suspect that he wants it for idol worship? Perhaps he wants it to build his house. On this the Mishnah teaches that with all other items, if he asked in a general way - "I would like a chisel" - it is permitted to sell it to him. But if he spelled it out and said, "I want a chisel in order to make an idol with it" - the sale becomes forbidden.
Rabbi Meir's addition:
Rabbi Meir adds further items to the list enumerated above:
High quality dates (the dates themselves, not the date palms).
Sugar cane.
Another type of costly plant, whose precise identity is unclear.
Here too the concern is that these items were commonly used for idol worship, and therefore it is forbidden to sell them to idol worshippers.
In practical halachah: This has no practical application in our times. However, a person living in a place where idol worship is widespread, and where there is a particular item used specifically for that worship, is indeed subject to this restriction and may not sell it to those who worship there. For example, someone living in India, where butter is offered to idols, would be restricted in selling butter, and so on.