Gemara

(Mishnah): There are four laws of (retraction from) a sale. If Reuven sold to Shimon (what was understood to be) good wheat, and it was found to be bad, Shimon can retract. If he sold bad wheat, and it was found to be good, Reuven can retract. If it was like it was understood to be, neither can retract.

In the following cases, either can retract. He sold red wheat, and it was found to be white, or olive wood, and it was found to be sycamore, or wine, and it was found to be vinegar, or (in any of these cases) vice-versa.

(Rav Chisda): If Levi sold to Yehudah something worth five for six, and its value increased to eight, only Yehudah can retract, for he was overcharged. He can say 'had you not overcharged me, you could not retract. Now that you overcharged me, can you retract?!'

Support (Mishnah): If Reuven sold good wheat, and it was found to be bad, Shimon can retract.

Bava Metzia 50b (Beraisa - R. Noson): If the Ona'ah is more than a sixth, the sale is void.

Rebbi says, whoever was deceived has the upper hand. He can void the sale, or demand payment of the Ona'ah.

Rishonim

The Rif and Rosh (5:13) bring the Mishnah.

Ran (DH Yafos): When it was good (or bad) like he sold, what is the Chidush that neither can retract? Rav Hai Gaon says that even if they were found to be the best that there are, the seller cannot say that he was underpaid, since he sold good. Even if they were found to be the worst that there are, the buyer cannot say that he overpaid, since he bought bad. The Rambam says that the Chidush is, even if it was not the worst or best that there is, neither can retract. Both of these are true.

Rambam (Hilchos Mechirah 17:1): There are four laws of sales. If Reuven sold good wheat, and it was found to be bad, Shimon can retract. If he sold bad wheat, and it was found to be good, Reuven can retract. If he sold bad wheat and it was found to be bad, or he sold good wheat and it was found to be good, even if it was not the worst or best that there is, and there is Ona'ah of a sixth, neither can retract. He acquires, and the Ona'ah is returned.

Hagahos Maimoniyos (1): The same applies if he sold Stam (neither can retract, even if it was found to be good or bad).

Magid Mishneh: If he sold good and it was found to be bad, even if the difference in value is less than the Shi'ur of Ona'ah, Shimon can retract, for this is Shinuy Shem; it is like a small change in the item. Reuven cannot retract, for it is not a totally different item. Reuven can retract if he sold wheat and Shimon told him that it is bad, and it was found to be good.

Rambam (2): However, in the following cases, either can retract. He sold red wheat, and it was found to be white, or olive wood, and it was found to be sycamore, or wine, and it was found to be vinegar, or vice-versa. Either can retract, for it is not the species he sold. The same applies to all similar cases.

Hagahos Ashri (Bava Metzia 2:9): A case occurred in which Levi bought (what was assumed to be) tin from a Nochri smith, in order to cover his roof. He sold it to David. It was found to be silver covered with tin. The Re'em exempted David, for Levi did not acquire the silver, since he did not know about it. R. Tam agreed.

Mordechai (Bava Metzia 291): Reuven claims that Shimon sold him a golden item and said that it is gold, and he broke it and found that it is of tin. He demands his money back, for the Ona'ah is above the Shi'ur and it is Bitul Mekach. Shimon says 'I did not promise anything to you. I sold to you like I bought it. Also, when I bought it from the Nochri who swindled me, you wanted to buy it. When I sold it to you, I did not know that I swindled you. If you will swear that I promised that it is of gold, I will return your money.' Rabbeinu Meir said, even though he did not promise that it is of gold, since externally it looks like gold, and the price proves that he paid for gold, it is a Mekach Ta'os. Even though Devarim sheb'Lev (unspoken intentions) have no effect, and he did not say that it is gold, it is as if he said it, for we know that the sale was based on the assumption that it is gold. This is like one who sold, and he did not need the money. Even though he sold Stam, it is a Mekach Ta'os and the item and money are returned. In many such cases, Devarim sheb'Lev count. However, if Shimon does not believe that the inside was tin, he swears that he does not know that there was tin inside, and he is exempt. Even though the Gemara did not answer whether there is a time limit to claim Bitul Mekach due to Ona'ah above a sixth, here it is surely Mekach Ta'os, like one who bought red and it was found to be white. There, some prefer white. Here, all prefer gold!

Poskim

Shulchan Aruch (CM 233:1): If Reuven sold a kind of Peros and gave a different kind, there is no sale. Either can retract. E.g. if he sold white wheat, and it was found to be red, or wine, and it was found to be vinegar, or olive wood, and it was found to be sycamore, or vice-versa.

SMA (1): Even if what he received is worth more or less than what was supposedly sold, either can retract, for it is a different species. However, if he sold good and it was found to be bad, since it is the same species and all prefer good, only the buyer can retract. If he sold bad and it was found to be good, only the seller can retract.

Shulchan Aruch (ibid): However, if he sold good wheat, and it was found to be bad, Shimon can retract, even if there was no Ona'ah regarding the amount. Reuven cannot retract, even if the price rose.

SMA (2): This is because some want to buy only good, even if it is expensive, and not bad, even if it is cheap.

Shulchan Aruch (ibid): If he sold bad wheat, and it was found to be good, even if there was no Ona'ah regarding the amount, Reuven can retract. Shimon cannot retract, even if the price dropped. If he sold bad wheat and it was found to be bad, or he sold good wheat and it was found to be good, even if it was not the worst or best that there is, and there is Ona'ah of a sixth, neither can retract. He acquires, and the Ona'ah is returned.

SMA (4): When worse ones are found, one might have thought that the buyer can retract and say 'since these are not the ones I bought, I retract', for any reason that he wants to retract. We learn that this is not so.

Nesivos ha'Mishpat (Bi'urim 2): When worse ones are found, one might have thought that the seller can retract!

Rema: If he sold meat assumed to be from a castrated ram, and it was found that it was not castrated, the sale stands and he returns the Ona'ah, unless it is known that the buyer is finicky and eats ram meat only if it was castrated. The same applies to all similar cases.

Source: Terumas ha'Deshen 322, cited in Beis Yosef DH Kasuv.

Gra (3): This is even though most people have no preference, like we find in Bava Basra 23a (the Halachah is concerned for what finicky people are concerned for).

Rema: If he sold refined silver, and it was found to have impurities, the sale stands and he returns the Ona'ah, for it is all the species of silver.

Beis Yosef (DH v'Chosav): Avi'asaf says that he cannot retract unless there is a Shi'ur of Ona'ah (for Bitul Mekach).

Question (Shach 1): Why is this different than a sale of good, and it was found to be bad? Perhaps the Rema means 'do not say that either can retract, rather, only the the buyer can retract.' Perhaps the text should say 'the seller cannot retract.' Ra'avan explicitly says so. Rashdam says that Avi'asaf is a lone opinion.

Answer (Gra 4): Avi'asaf holds that in our Mishnah, the buyer can retract due to Ona'ah. This is like R. Noson (it seems that this should say 'Rebbi', or 'Rebbi, who argues with R. Noson'), who says that he has the upper hand. Rav Chisda supports this, but the Rashbam says that all agree to our Mishnah, and the Rambam says that our Mishnah is even when there is no Ona'ah.