Gemara

(Mishnah - R. Yehudah): If one sells a Sefer Torah, animal or pearl, the buyer cannot claim Ona'ah (that he was overcharged);

Chachamim say, only the exceptions listed (see the coming Mishnah) are not subject to Ona'ah.

Bava Metzia 56a (Mishnah): Ona'ah does not apply to the following: slaves, documents, land, and Hekdesh.

57a Question (R. Ami): Does Bitul Mekach apply to them?

(Rav Nachman): He later concluded that it applies.

(R. Yonah citing R. Yochanan): Ona'ah does not apply to Hekdesh, but Bitul Mekach applies.

(R. Yirmeyah citing R. Yochanan): Ona'ah does not apply to land, but Bitul Mekach applies.

R. Yonah says so about Hekdesh, and all the more about land (in which the buyer and seller erred)! R. Yirmeyah said so about land, but regarding Hekdesh, the sale is never invalid;

(Shmuel): If 100 Maneh were redeemed on a Perutah, the redemption works.

58b (Beraisa - R. Yehudah): Ona'ah does not apply even to a Sefer Torah, because it is priceless. It does not apply to an animal or pearl, because people want (and willingly pay extra for) a good match of animals or pearls;

Chachamim: People want a good match for everything!

R. Yehudah: These are more important to people than other things.

Question: Up to how much is there no Ona'ah?

Answer (Ameimar): It is until double their value.

(Beraisa - R. Yehudah ben Beseira): Also if one sells a horse, sword or shield in wartime, Ona'ah does not apply, for one's life depends on them.

108a - Assumption: If land worth 100 was sold for 200, Levi (the neighbor, who has first rights to buy it) tells the buyer 'you were like a Shali'ach to buy the land for me, but not to overpay! I need to pay you only 100.'

Rejection (Mar Kashisha): Chachamim of Neharda'a say in the name of Rav Nachman that land is worth whatever one pays for it. (Levi must pay 200.)

Kesuvos 100b - Question: Up to how much can Beis Din err (and the sale stands)?

Answer (Rav Huna bar Yehudah): They can err up to half the value.

Support (Beraisa - R. Shimon ben Gamliel): If Beis Din sold a field worth 200 for 100, or a field worth 100 for 200, the sale stands.

Rishonim

Rif (Bava Metzia 32a): The Halachah follows Rav Nachman, who is Basra. T'here is no Ona'ah even when the price is double (Bava Metzia 108a).

Rebuttal (Rosh Bava Metzia 4:21): The Halachah follows the latter Chacham only from Abaye and Rava and onwards! We follow R. Yochanan against Shmuel, and all the more so against Rav Nachman, Shmuel's Talmid.

Rif (32b) and Rosh: Some say that since our Sugya strove to explain R. Yehudah, this shows that the Halachah follows him. It also follows R. Yehudah ben Beseira, who follows estimation (of intent). Rav Hai Gaon says that the only exceptions are what the Mishnah says (slaves, documents, land and Hekdesh).

Rambam (Hilchos Mechirah 12:8): Ona'ah applies to animals.

Rambam (13:8): Ona'ah does not apply to slaves, documents, land and Hekdesh, even if one sold something worth 1000 for one or vice-versa.

Rambam (Hilchos Malveh 12:11): If Beis Din properly announced (about a field they are selling) and investigated the price, if they sold land worth 100 for 200 or vice-versa, the sale stands.

Magid Mishneh: The Ramban says that if land worth less than 100 was sold for 200, it is Batel.

Tumim (66:65): The Magid Mishneh brought the Ramban without saying that he argues with the Rambam. Surely the Ramban holds like the Rambam, that normally there is no Bitul Mekach even less than half. Also, Milchamos Hash-m defended the Rif! Beis Din is authorized to sell only the way people often buy, i.e. up to half. We must say that there is no Bitul Mekach at exactly half.

Tosfos (Bava Basra 61b DH Shma): The Ri says that all agree that the price of land does not prove how much was sold, for people sometimes pay more than land is worth, like it says in Kesuvos (91b). The Rashbam says that it is because Ona'ah does not apply to land (it is worth whatever one pays for it). This is wrong. Ona'ah does not apply to slaves and Hekdesh, even though they are not always worth what one pays for them! Rather, it is a Gezeras ha'Kasuv. Also, Bitul Mekach applies to them (all things for which there is no Ona'ah)!

Poskim

Shulchan Aruch (CM 227:15): Ona'ah does not apply to slaves, documents, land and Hekdesh, even if one sold something worth 1000 for one or vice-versa.

Beis Yosef (Sof DH v'Elu): Since the Rif and Rambam agree, we rule like them.

Rebuttal (Darchei Moshe 4): I do not agree that we follow the Rif and Rambam when the Rosh and Tosfos disagree, especially since the Tur agrees with the Rosh and Tosfos. In any case, ha'Motzi mi'Chavero Alav ha'Re'ayah.

Beis Yosef (DH Kosav Rabbeinu): R. Yerucham says that Rav Hai Gaon says that Ona'ah does not apply to 'Hekdesh' for Aniyim or for the Beis ha'Keneses, and the Rif disagrees. R. Yerucham holds like Rav Hai Gaon. The Tur holds like the Rif.

SMA (49 and Drishah 40): The Tur, and the Rif and Rosh (in their commentary on the Gemara) did not mention Hekdesh. The Beis Yosef derives from this that the Rif holds that Ona'ah applies to Hekdesh Aniyim, for nowadays this is the only Hekdesh, and it is Chulin (Teshuvas Rosh 13:1). We exclude Hekdesh because it is not "Amisecha" (your fellowman). Hekdesh Aniyim is of your fellowman, so Ona'ah applies.

Rema: Some say that this is only until half, but more than half, e.g. he sold something worth one for more than two, there is Ona'ah.

SMA (50): Indeed, Tosfos cited R. Tam to say that Ona'ah applies to land above a half, but this was merely to argue with the Rif and Rambam. Really, it applies also at exactly a half, like the Rosh and Tur say!

Shach (17): The Rema is precise, like Tosfos (Bava Kama 14b DH Davar) and the Agudah. The Yerushalmi was imprecise! Tosfos (Kidushin 42b DH Hachi) says that either the Yerushalmi (which says that there is Ona'ah at a half) is imprecise, or Bava Metzia 108a is imprecise (the land was sold for less than 200). However, I hold that the Rif's opinion is primary. (There is no Ona'ah even past a half.)

Gra (41): The Gemara in Bava Metzia 108a, 58b and Kesuvos 100b connote that Ona'ah is only at less than a half.

Tumim (65): The simple reading of the Gemaros connotes that Ona'ah is only at less than a half. This mimimizes arguments among the Meforshim. Also, if a commoner's sale (of land worth 100) is valid only if the price was up to 199, why did Chachamim enact that Beis Din's sale is valid up to one more (200)? Rather, surely, they made Beis Din like commoners.

SMA (51): The Maharshal says that even though Ona'ah does not apply to these things, Ona'as Devarim and Geneivas Da'as apply, unless the seller himself was unaware.

SMA (52): 'Ona'ah' in the Rema refers to Bitul Mekach.

Gra (40): This opinion explains Bava Kama 14b to say that land is worth all the money you pay for it to a greater extent than Metaltelim are. R. Yehudah taught that Ona'ah does not apply even to a Sefer Torah, i.e. it has the same law as land. The Gemara explained that Ona'ah does not apply to a Sefer Torah until double the value.