Gemara

(Beraisa): If one sells his Olah or Shelamim, it does not take effect.

Bava Kama 78a - Question (Rava): If Reuven said 'Alai Olah', and he separated an ox for this, and it was stolen, can the thief exempt himself through returning a smaller animal Kosher for an Olah?

Perhaps Reuven can say 'I intended to do an ideal Mitzvah (a big Korban. Therefore, you must return a big animal.)'

Answer (Rava): The thief can exempt himself with a lamb.

Rishonim

Ran (Chulin 14a DH Lo): Seemingly, if one bought an animal that we consider Terefah due to Safek, it is not a Mekach Ta'os. The seller can tell the buyer 'prove to me that it is Terefah, and I will return your money.' There was a case of Hagramah (Shechitah that left a ring of the windpipe) in which Rav said that it is Terefah, and exempted the Shochet, for he was unsure if the Halachah follows R. Yosi b'Rebbi Yehudah or Rabanan. The Shochet can say 'prove to me that I am liable, like Rabanan, and I will pay.' Seemingly, here is similar. However, this is wrong. We cannot obligate the Shochet for his action unless it is Vadai Terefah. Here, people refrain from a Safek [Terefah], or what we forbid from a tradition from the Ge'onim, so this is the ultimate blemish, so it is a Mekach Ta'os.

Maharam Mintz (113): If Reuven bought a big Esrog for Hidur Mitzvah for a high price, and Shimon destroyed, disqualified or took it, it suffices for Shimon to give to him a Kosher Esrog. It need not be Mehudar. We learn from a thief. He exempts himself with a lamb.

Tosfos (Chagigah 13a DH Ein): A Nochri who learns Torah is Chayav Misah, and a Yisrael who teaches him transgresses "v'Lifnei Iver Lo Siten Michshol." This does not apply to his seven Mitzvos. It is a Mitzvah to teach them to him!

Tosfos (Kesuvos 40a DH Neisi): When a raped woman is forbidden to the rapist, why do we need a verse to teach that there is no Mitzvah to marry her? If she does not want to marry him, there is no Aseh. Therefore, even when she wants, it is not considered an Aseh to be Docheh a Lav! Really, the verse is not extra.

Tosfos (Bava Metzi'a 10b DH d'Amar): A Yisrael Shali'ach who is Mekadesh a divorcee to a Kohen transgresses Lifnei Iver.

Ritva (Sukah 10b DH Rav): Even though the Reish Galusa did not know that Rav Chisda and Rabah bar Rav Huna retracted from their teaching, or that they were Sheluchei Mitzvah, he set them to sleep in a Sukah that he considered to be Kosher [even though the decorations were four Tefachim below the Schach]. He was not concerned that they consider it a Pasul Sukah, and they bless there improperly. This is like putting a stumbling block in front of a seeing person. Some derive that if one holds that a food is permitted, Lifnei Iver does not apply even if he feeds it to one who forbids it, and the latter is a Ba'al Hora'ah (authorized to rule in Halachah). The feeder is also a Ba'al Hora'ah. He may rely on his opinion to feed to others. I say that the case of Sukah was different, for the Isur was evident to the recipient. If he holds that it is forbidden, he need not eat it! This does not apply if it is not evident. Rav said (Chulin 111b) "Heaven forbid that Shmuel would serve me something that I forbid!" My Rebbi ruled like this. However, if the Halachah was decided unlike him, it is forbidden for him and others.

Poskim

Shulchan Aruch (CM 232:12): Terefos of lesions cause Bitul Mekach. Some disagree.

Rema: Even a Terefah that we forbid due to Safek is Mevatel Mekach. The seller cannot say 'bring a proof that it is Terefah.'

SMA (28): The Ran says that the same applies to Terefos of stringencies of the Ge'onim. It seems that also the Rema refers to cases of Safek, i.e. not all agree that it is Terefah. If not, what is the Chidush? Also Trefos of lesions are due to Safek, for we are not experts to check!

Gra (21): This is unlike Hagramah. Since one may not eat [Safek Terefah], it is a Mekach Ta'os.

Nesivos ha'Mishpat (Chidushim 12): The same applies to a mere stringency [not due to Safek].

Eliyahu Rabah (656:1): If Shimon disqualified Reuven's Esrog, he must pay only a small Esrog Kosher for the Mitzvah.

Rebuttal (Mishneh l'Melech Hilchos Ma'aseh ha'Korbanos 16.7 and Chacham Tzvi 120): We cannot learn from Bava Kama. The ox had no Damim (price), for one cannot sell his Olah or Shelamim. Here, Reuven could have sold his Esrog for the same price he bought it for!

Yad Avraham (YD 62:2): Emunas Shmuel (10) says that one does not transgress Lifnei Iver when he himself is not commanded about the matter. The Pri Chodosh (YD 62:3) and Taz (1, according to Mishbetzos Zahav) and Tosfos (Bava Metzi'a 10b) disagree. Tosfos in Chagigah (13a) proved that even if one has a Mitzvah to do an action, Lifnei Iver applies when he induces someone who is forbidden to do it, e.g. he teaches Torah to a Nochri. We learn also from Tosfos in Kesuvos (40a). If not, Tosfos should have said that we need the verse for one who marries her!

Yabi'a Omer (5 CM 6): If one accidentally put food under his bed and slept on it, and sold it, and the buyer found out afterwards, may he return it? Many Poskim forbid the food even b'Di'eved. He is concerned for them. The seller says 'I am sure that the Halachah follows the lenient opinions.' Seemingly, this is like the Ran wrote about a Safek Terefah. The Rivash (499) and Rema wrote similarly. Poskim argue about food under a bed. A person does not enter a Safek [willingly]. However, the Ran and Rivash discuss when the Poskim forbid, e.g. Safek Terefah. We are stringent about a Safek Torah law. Therefore, the buyer can demand his money back. Here, most Poskim permit b'Di'eved, and mid'Oraisa there is no concern, so we are lenient, especially to avoid a big loss. Therefore, one cannot take from the seller, who is Muchzak in his money. This is like an animal that a Chacham needed a new reason to permit it. It is Midas Chasidus not to eat it. We do not obligate the seller due to the buyer's Midas Chasidus. Most people are lenient b'Di'eved [about food under a bed], so it is not a blemish to be Mevatel the sale.

Yabi'a Omer: The Chasam Sofer (65) ruled that if a matter was permitted to Reuven to avoid a big loss, it was permitted only to him. Others may not buy from him l'Chatchilah. Sha'arei De'ah (1:154) questioned this. R. Yom Tov Algasi (Hilchos Yom Tov, Bechoros 5) brings from the Radvaz (Hilchos Ma'aser, 12:18) that a buyer cannot rely on his Midas Chasidus to claim from the seller. Bava Kama 78b supports this. The owner's desire to bring a big Korban does not obligate the thief money. Maharam Mintz learned to a case of a Mehudar Esrog. The Chacham Tzvi and Mishneh l'Melech refuted this, for one cannot sell a Korban. This rejects Sha'arei De'ah's proof from Bava Kama. In any case, it is Midas Chasidus for the seller to inform the buyer. Perhaps he does not want to buy something that Acharonim argued about! Similarly, if one paid much for Mehudar Tefilin, if a Rav needed to rule about a question about their Kashrus, even he ruled leniently, the seller must tell the buyer. If he does not, perhaps this is Ona'ah (deception). The buyer has a claim if the Heter is not clear according to most Poskim.

Yabi'a Omer (3): Keren l'David (OC 120) forbids putting into wine Stam sugar (that was not guarded), for some are stringent about this and pay much for Pesach wine. Perhaps it is Ona'as Mekach, and Lifnei Iver, if the buyer is stringent about this. Beis Meir says that it is a mistaken custom, for people did not know how sugar is made. She'alas Ya'avetz (2:65) says that sugar is more Kosher than our Matzah for the Seder! However, some Acharonim are stringent.

Yabi'a Omer (4): A case occurred in which Reuven betrothed Leah, and later found out that her father is named Reuven. The Tzava'ah of R. Yehudah ha'Chasid forbids such a marriage. Toras Chesed (268) ruled about David who betrothed his brother's wife's sister, and later found out that the Tzava'ah forbids brothers to marry sisters. He permitted David to retract without any fine. This is Ones, for the Shiduch will not go well. There is no danger in the matter, but many are particular, and for them there is reason for concern. Also, there he is Muchzak, and seeks to exempt himself. If a buyer did not yet pay, he can return the item because he is concerned for the stringent opinions, especially if we know that he is normally careful