Gemara

(Mishnah): A caravan of donkey-drivers or camel-drivers can save a city from becoming an Ir ha'Nidachas (if they lodged there 30 days, and they help make a majority that did not serve idolatry).

(Beraisa): If a caravan of donkey-drivers or camel-drivers stopped in a city that became an Ir ha'Nidachas (and they were enticed with it), if they were in the city for 30 days, they are killed by the sword and their property is destroyed (like residents of the city). If they were there less than 30 days, they are stoned, and their heirs inherit their money (like individuals who served idolatry).

Contradiction (Mishnah): One who has lived in a city 12 months is considered a resident. (He helps pay for city expenses.)

Answer (Rava): One is not considered to be (a person) of the city until 12 months. Regarding Ir ha'Nidachas, it says "Yoshvei (those who dwell in) ha'Ir" - after 30 days, one is considered to be dwelling there.

(Beraisa): A vow from 'people of a city' applies to people who have lived there for 12 months. A vow from 'Yoshvei ha'Ir' applies to people who have lived there for 30 days.

Bava Basra 7b (Mishnah): One who has lived in the city 12 months is considered a resident. If he bought a house, he is a resident immediately.

8a - Contradiction (Beraisa): After (living in a city for) 30 days, one must contribute to the Tamchuy (food for the poor of the city). After three months, he must give (money) to the Kupah (for the poor). After six months, he must contribute for their clothes. After nine months, he must contribute for their burial. After 12 months, he must contribute for planks (used to support the wall).

Answer (Rav Asi): Indeed, the Mishnah teaches that after 12 months he is a resident regarding planks.

Rishonim

Rif and Rosh (Bava Basra 1:23,24,27): If one owns a Chatzer in another city, people of the city can force him only to help dig pits and irrigation ditches. If he dwells in that city, people of the city can force him for everything. People of the city can force each other to build a Beis ha'Keneses and buy a Sefer Torah, Nevi'im and Kesuvim. After living in a city for 30 days, three months, six months, nine months and 12 months, respectively, one must contribute to the Tamchuy, Kupah, clothes of Aniyim, burial, and planks. We learn that we force people to pay for all of these.

Nimukei Yosef (DH Masnisin): It is clear from the Gemara that even if he did not intend to settle in the city, since he was there 12 months, he is like a resident for all taxes, conscription, planks and all needs of the city. If he was not there for 12 months, even if he came to settle, he is like a resident only for Tamchuy, Kupah, clothes of Aniyim and burial, after the set times. Buying a house makes one a resident only if he lives there. If not, he is a resident only for things that benefit the house, e.g. digging pits and irrigation ditches. This helps the house, for it promotes settlement of the city. Renters will be available, and if he wants he can sell it for more. If he lives there, he is liable like all residents. He is exempt (from what does not benefit his house) not only if he bought, in which case his fathers were never liable. Rather, even if he always had a house there, since he does not live there he is exempt, even though he could live there.

Rambam (Hilchos Shechenim 6:1): People of the city can force each other to build a wall, doors and bolt, and a Beis ha'Keneses, and buy a Sefer Torah, Nevi'im and Kesuvim for anyone in the Tzibur who wants to read.

Rambam (3): If one owns a Chatzer in another city, people of the city can force him only to help dig pits and irrigation ditches. If he dwells in that city, people of the city can force him for everything.

Rambam (5): If one lived in the city for 12 months, or bought a house in the city, he gives with people of the city for everything that must be fixed: the wall, doors, wages of the horsemen and guards, and all that guards the city.

Poskim

Shulchan Aruch (CM 163:1): People of the city can force each other (Rema - even the minority can force the majority) to build a wall, doors and bolt, and a Beis ha'Keneses, and buy a Sefer Torah, Nevi'im and Kesuvim for anyone in the Tzibur who wants to read.

SMA (1): This connotes that even people far from the border must pay for the wall. Here the Shulchan Aruch adopted the text of the Mishnah. Sa'if 2 explains who is considered a resident.

SMA (2): We have Seforim among us, so we force only to buy a Sefer Torah. Beis Din can force people to lend their Seforim to avoid Bitul Torah.

Rema: The same applies to all needs of the city. People of the city can force each other to host guests, and give Tzedakah to them and to the Tzedakah bag.

Shulchan Aruch (2): If one owns a Chatzer in another city, people of the city can force him only to help dig pits and irrigation ditches. If he lived in the city for 12 months, or bought a house in the city, he gives with people of the city for everything that must be fixed: the wall, doors, wages of the horsemen and guards and all similar things that guard the city.

Rema: This is Stam. If we know that he wants to settle in the city, immediately he is like people of the city. Some say that if he rented a house for 12 months, immediately he is like people of the city.

Gra (11): Rental is a temporary purchase. The Mishnah says 'bought' to excluded one who inherited or received a gfit.

Rema (ibid.): Some say that this is only if he bought to stay there. Some say that it is even to dwell (temporarily). All agree that inheriting or receiving a house for a gift does not make one a resident. However, if there is a custom, we follow it.

Beis Yosef (DH v'Chosav): The Rivash (475) says that the Mishnah discusses one about whom we did not know whether he will settle here. After 12 months, we obligate him in everything from now and onwards if he goes from place to place, like donkey-drivers who pass by. Anyone who was obligated from the beginning to live here for 12 months is immediately a resident, for presumably he intends to settle. Some say that even if he did not buy a house, but rented one for 12 months, he is immediately a resident. The Rivash (214) does not distinguish between one who bought a house before or after coming to live here. The Ritva says that the Mishnah says 'bought', but not 'if he has a house in the city' to exclude one who inherited or received for a gift. Since he did not toil for it, it does not show his intent.

Gra (21): The latter opinion learns from donkey-drivers. The Gemara asked that they were not there 12 months. It did not ask that they do not intend to settle (for this is not required).

SMA (7): The Ran (Teshuvah 11) says that marrying a woman in a city does not show that he intends to live there.

Rema (ibid.): Some say that this is for fixed taxes, but for a tax that is not fixed, once one rents a house he is liable like other residents. We can say that the tax is bigger due to him! Some say that living 12 months does not make him a resident when it was due to Ones, e.g. he became sick. However, if he profited, he must give according to his profit.

SMA (10 and Shach 4): Pregnancy is not an Ones. Women often ride and travel in the seventh or eighth month. If he claims that they authorized him to stay (and remain exempt from taxes), he need not swear, but they may impose a Cherem. If people of the city claim that Ploni lived there or did business there, they must swear or bring a proof. If not, Ploni swears to exempt himself.

Rema (ibid.): If one lived in a city for many years and they did not demand taxes from him, this does not show they pardoned it. He must pay retroactively for as long as he was liable.

Gra (28): Silence does not show that one pardoned. One cannot make a Chazakah against people of the Reish Galusa's house (Bava Basra 36a; they need not protest, for they can retrieve their land whenever they want). People of the city are like the Reish Galusa's house.

Rema (ibid): Once a tax was imposed on a city, one cannot exempt himself from it by leaving. Even if he says that he gave his share to the official, if the official denies getting it, he must pay again.

SMA (11): The Mordechai obligates him even if he left to go to Eretz Yisrael. Terumas ha'Deshen (342) obligates one to give for a new tax imposed on the city within 30 days after he left.