Commentary for Avodah Zarah 31:8

"To the Persians, who protect us" - It seems that we are also permitted nowadays to sell weapons to gentiles from this reason.

This baraita prohibits even selling defensive shields to non-Jews (although there is a version that allows one to sell them shields).
If it is prohibited to sell them weapons of defense, then it should also be prohibited to sell them food. After all, food is necessary to fend off death. And indeed, this is what Rav holds. If it would be possible to avoid selling food to them, then one should, according to Rav. I should note that this is an extreme opinion—the notion being that one should do whatever one can to avoid aiding non-Jews. It is not representative of the majority of the sages.

Another way of understanding why one should not sell them shields is that shields are easily turned into weapons. Indeed, while I’ve never battled with swords or shields, hitting your opponent with the shield seems to occur quite frequently in television and the movies.

Those who allow Jews to sell them shields explain that when they have no more weapons, the non-Jews will not use their weapons as shields, they will simply run away.
Nahman in the end allows selling them shields. As often occurs in these issues, the Talmud adopts the more lenient view.

Adda b. Ahavah adds that Jews may not even sell any raw material that would normally be turned into weapons. This refers only to a particular type of iron that would only be used for weapons. Other materials that may be used for other purposes may be sold to them.
Interestingly, the sugya again notes that this prohibition is not observed. Jews were selling material to non-Jews that could be used as a weapon. R. Ashi explains that such material is sold to Persians who protect the Jews.
This sugya has some interesting ramifications for the question as to whether or not it is permitted to sell weapons. One way of reading the sugya is that it is prohibited to sell weapons only to those who might use them against Jews. Jews (Israel) can sell weapons freely to others who fight each other. But the sugya might possibly be read more expansively as prohibiting selling weapons to anyone who would use them offensively.

Judah allows Jews to sell maimed animals to non-Jews because they cannot be cured. They will not be able to use them as work animals. The non-Jew will assumedly slaughter them immediately and it is not prohibited to sell animals used for food to non-Jews. The other sages point out that these animals can give birth. Thus a non-Jew may come to hold on to them. Another Jew would then see the non-Jew in possession of the animal and come to believe that it is permitted to sell animals to non-Jews.
Judah responds that maimed female animals cannot be bred. The non-Jew will see this very quickly and will not hold on to the animal for long.

The rabbis have a notion that carrying a person is not considered forbidden labor on Shabbat because a “live person carries himself.” Since a horse is used for riding, Ben Batera says one may sell a horse to a non-Jew because the horse does not perform labor forbidden on Shabbat from the Torah.
Rabbi [Judah Hanasi] forbids selling horses for two reasons. First of all, they are weapons. Second of all, they are considered large animals.

The prohibition of selling weapons applies to a horse, for a horse can kill by trampling. Indeed, it seems quite clear that horses are used as weapons. But the prohibition of selling large cattle is more difficult to explain, for as I said above, the rabbis do not consider carrying a human to be prohibited by the Torah. R. Yohanan answers that when the animal grows old it will be made to work in a mill, which is considered work.

Can a Jew sell a fattened ox to a non-Jew? The Talmud explains that this question can be asked of both R. Judah, who allows selling a maimed animal and of the rabbis who do not.

This source describes the House of Rabbi [Judah Hanasi] basically bribing the Roman authorities so that they would not have to give a fatted ox to be sacrificed. The second bribe they paid was to allow them to give them a dead ox, which obviously cannot be sacrificed. Now, since they already bribed the Romans that the ox should be presented the day after their festival, what does it matter whether the ox is dead or alive? The answer has to be that they were concerned lest they hold on to the fatted ox and use it for labor prohibited on Shabbat.

The problem with the above explanation is that if the animal was dead, then why would Rabbi also have paid for it to be presented the day after the festival. The baraita simply makes no sense read that way.
Rather, the series of payments was just a gradual way to ease the Romans into allowing Rabbi and his house not to have to give the fatted ox altogether.

Ashi affirms that if a fatted ox is kept and slimmed down it can eventually perform the work of two. This is why it is problematic to sell them a fatted ox.

A Jew is not allowed to sell bears or lions to non-Jews, lest the non-Jew not take proper care of the animal and the animal cause damage to the public. This section may also be understood as referring to the gladiator exhibitions which often involved lions and bears. The Rabbis were opposed to these violent exhibitions and to the culture that encouraged the enjoyment of bloodshed. A Jew should certainly not attend such events nor sell animals to non-Jews that might be used in these events.

A Jew may not join non-Jews in building any of the structures listed in this section. The basilica served as a courtroom. The scaffold was used for executions. The platform was used for judges to make speeches. We can see from this mishnah that the Jews had a deep distrust for the non-Jewish law system. This law system is considered damaging to the public and therefore a Jew should not aid in building this system.

A Jew is allowed to join non-Jews in building bathhouses, since bathhouses are for the public good. Indeed in other Talmudic sources the Romans are praised for building bathhouses, marketplaces, roads and bridges. From other stories in the Talmud and a famous story that will appear in our tractate, it is clear that Jews and non-Jews shared public bathhouses.
[Note: the version of the mishnah upon which my interpretation is based is “dimasaot” and not “bimasaot”, as recommended by Albeck.]
A Jew may not aid in building the cupola of the bathhouse since that is the place where the idol is placed. As we have learned already in the previous mishnayoth, a Jew may not perform any action which would aid in idolatrous practice.

“Struggling” refers to an animal that was dying before it was slaughtered. If after it was slaughtered it “struggles” then we can assume that it was alive and the animal may be eaten. R. Hanin here is discussing the status of large wild animals. All agree that their status vis a vis struggling is the same as that of a small beast. But when it comes to selling them to non-Jews, there is a dispute. The first opinion holds that large wild animal may never be sold. In contrast, R. Hanin holds that like small beasts, it depends on the custom.

The Talmud raises a difficulty on anyone who says it is prohibited to sell them large wild beasts. Large wild beasts may not be sold only if they present a danger to the public. But if they are not dangerous, then they may be sold to them.

Rabbah b. Ulla explains that the Mishnah prohibits selling a mutilated lion which is not fit for work, and it follows the opinion of R. Judah from the previous mishnah who forbade selling maimed animals. But an unmaimed large wild animal would be prohibited, even according to the other rabbis, even if it does not present a danger to the public.
Ashi says that all lions are considered “maimed” when it comes to work. After all, a lion cannot be put to work. Thus a lion that does not present a danger could be sold. But a different wild animal that can do work cannot be sold under any circumstance.