Gemara

20b (Mishnah): If one set up a store in a Chatzer, others can stop him because the noise of people entering and leaving disturbs their sleep. Rather, he may make Kelim in the Chatzer and sell them in the market. If noise of his hammer, millstone or the children disturbs their sleep, they cannot complain.

Question: What is the difference between the clauses?

Answer #1 (Abaye): The Seifa teaches that people of another Chatzer cannot complain.

Objection (and Answer #2 - Rava): If so, it should say so explicitly! Rather, the Seifa discusses one who teaches Torah to children.

21a - Question (Beraisa #1): People of a Chatzer can prevent a member from setting up practice as a doctor... or a teacher of children.

Answer: That refers to teaching Nochri children.

21b (Rav Huna): If a member of a Mavoy set up a mill (to grind for wages, or to rent it out), he can stop someone else in the Mavoy from setting one up, for this infringes on his livelihood.

Question (Beraisa #2): One may set up a store next to another's store, or a bathhouse next to another's bathhouse. The first cannot protest, for each is on his own property.

Answer: Tana'im argue about this.

(Beraisa #3): People of a Mavoy may force one another not to allow a tailor, tanner, teacher of children or a craftsman to set up shop among them. One cannot prevent his neighbor.

R. Shimon ben Gamliel says, he can prevent him.

(Rav Huna brei d'Rav Yehoshua): Surely, one can prevent someone from another city. If they both pay head-tax in the same city, he cannot prevent him. One cannot prevent someone from the same Mavoy.

Question (Rav Huna brei d'Rav Yehoshua): Can one prevent someone from a different Mavoy?

This question is not resolved.

(Rav Yosef): Rav Huna admits that one cannot prevent a teacher of children from entering the same Mavoy. Jealousy among Chachamim makes them learn better.

Rishonim

Rif and Rosh (2:12): The Halachah does not follow Rav Huna, for he holds like an individual (R. Shimon ben Gamliel). Also, Rav Huna brei d'Rav Yehoshua is Basra, and he argues with Rav Huna. Since the question was not settled, they cannot stop him.

Rosh: Rav Huna had not heard the Beraisa. Had he heard it, he would not have ruled like R. Shimon ben Gamliel against Chachamim.

Rambam (Hilchos Shechenim 6:8): People in a Mavuy can force each other not to establish a tailor, tanner or any other profession among them. If there was such a professional in the Mavuy and they did not protest, or there was a bathhouse, store or mill, and someone else made another bathhouse or mill opposite it, he cannot stop him and say 'you cut off my livelihood!' This is even if the latter is from another Mavuy, for they already have such a professional.

Tosfos (21b DH Kofin): The Ri explains that Beraisa #3 means that people of a Mavoy can prevent a teacher of Nochri children, but one cannot force his neighbor. We cannot say so if Beraisa #1 discusses within a Mavuy, for it allows stopping a member from teaching children, i.e. Nochri children! Here, even R. Shimon ben Gamliel forbids only due to competition! However, the text of Beraisa #1 discusses within a Chatzer. The Rashba says that Beraisa #3 allows stopping a teacher of Yisrael children, even after the enactment, i.e. when there is already a teacher there. Rav Yosef says that Rav Huna agrees about a teacher, and he holds like R. Shimon ben Gamliel! He must say that R. Shimon ben Gamliel does not argue about a teacher.

Gra (CM 156:16): People can protest against a teacher from another Mavuy from teaching in this Mavuy. We say that one cannot protest even against one from another Mavuy when the Mavuy is Mefulash, for in any case there is traffic, like the Rif says. Rav Huna asked about protesting due to competition in such a case, when increased traffic does not apply.

Poskim

Shulchan Aruch (CM 156:5): People in a Mavuy can force each other not to establish a tailor, tanner or any other profession among them. If there was such a professional in the Mavuy and they did not protest, or there was a bathhouse, store or mill, and someone else made another bathhouse or mill opposite it, he cannot stop him and say 'you cut off my livelihood!' Even if the latter is from another Mavuy, they cannot stop him, for they already have such a professional.

Beis Yosef (DH v'Chosav): The Ramban asks, according to Rashi, how could we say that one cannot protest about another Mavuy? If he is from another city, the entire city can protest! Perhaps he holds that it discusses one from another city who pays taxes here. Or, perhaps the question was about in the same Mavuy, but he can stop one from another city. Even though the latter could rent a house here, and then no one could stop him, right now others can stop him. This is wrong. Some say that we discuss one who seeks to enter the adjacent Mavuy. The first can say that he does not want a competitor in a Mavuy in which he is a partner. Also, people of the first's Mavuy will go to the latter's Mavuy. The Ran and Nimukei Yosef wrote like the first version. I answer for Rashi that the people of the city can protest about another Mavuy.

Beis Yosef (DH Nimtza): The Halachah is, if the latter is from the same city, the first cannot protest even in the same Mavuy.

SMA (16): The Rambam explains that the Beraisa allows people in a Mavuy to stop each other from setting up a store due to increased traffic. This must refer to a Mavuy she'Eino Mefulash. Even if all but one agree, the one can stop the others. This is why the Rambam (and Shulchan Aruch, who cites the Rambam verbatim) bring this law in the singular (he can stop). The Beraisa says that one cannot stop a neighbor, i.e. when there is already such a professional in the Mavuy. One cannot stop him in the same Mavuy, for he did not protest against the first. Even the first professional cannot stop him. One cannot stop him even in another Mavuy, for the question was not settled, so we are lenient for the defendant, i.e. the one who wants to enter. The Rambam holds that the Reisha discusses a Mavuy she'Eino Mefulash, and is due to increased traffic. The others consider a Mavuy she'Eino Mefulash like a Chatzer; there, even if there is already a professional, people can protest due to increased traffic.

Bach (10 DH v'Ika): The Beraisa mentions only a tailor, tanner or a craftsman, and so says the Rambam. Why did the Tur mention also a weaver or doctor? He explains that the Beraisa is not concerned for noise, rather, for competition. Therefore, all professions are the same. If so, what is the Tur's source that a Mavuy she'Eino Mefulash is like a Chatzer regarding noise? He holds like the text of the Rif and Rosh, that Beraisa #1 discusses a Mavuy, and allows people to protest. This refers to a Mavuy Mefulash; if it were not Mefulash, they would not have reason to protest. Abaye permitted in a Mavuy Mefulash.

Note: In our texts of the Rif and Rosh, Beraisa #1 discusses a Chatzer.

Gra (15): R. Shimon ben Gamliel and his first Tana argue about protest due to competition. The Halachah follows the first Tana. When the Beraisa discusses a neighbor, this must refer to when there is no professional in the Mavuy. If not, when does the Reisha say that he can stop him? Within the Mavuy, the Seifa says that he cannot stop him! We never resolved the question about outside the Mavuy. Anyone from the city (not only a neighbor) can stop someone from outside the city!

Rema: Many say that people of a Chatzer or Mavuy she'Eino Mefulash can protest, but the other professional cannot protest.

Shach (4): The Rema holds like the Beis Yosef said, that the Poskim other than the Rambam say that the Mavuy can protest against the second, even though they did not protest against the first. I say that perhaps they agree to the Rambam.

Chazon Ish (CM 21 Likutim for Daf 149): Income is fixed from Rosh Hashanah, just one must engage in wordly matters (Hishtadlus). It is improper for a Chacham to try to cause a loss to Rav Mari, even though had Isar died (without transferring the money), there would be no Chiyuv to give the money to Mari, even to make Chachamim happy (Tosfos 149a DH Migmarei). Perhaps Rava wanted to use the money for needs of the Tzibur. Mari was rich (Bava Metzia 73b); he would not have minded if the money was used for Tzedakah and Chesed, for he was a Talmid.