Gemara

90a (Shmuel): One may not profit (through buyng and reselling food) more than a sixth.

91a (Beraisa): One may not make a double profit on eggs.

(Mari bar Mari): Rav and Shmuel argue about this. One explains that one may not sell them for double (what he paid). The other explains that one may not buy and resell them to a second dealer. (He must sell to consumers).

Bava Metzia 40a: In Rav Yehudah's area, there were 48 Kuz in a barrel; the price was six Zuz for a barrel. Rav Yehudah would sell six Kuz for a Zuz. He would get back the six Zuz he paid after selling 36 Kuz of the barrel. Eight Kuz (a sixth) were absorbed in the barrel, his whole profit was four Kuz.

Question: Shmuel taught that one may not profit more than a sixth! (Rav Yehudah was entitled to take a bigger profit. Why didn't he?!)

Answer: He would also keep the barrel itself and the dregs (he received 48 Kuz of clear wine and dregs for six Zuz).

Question: If so, he profited more than a sixth!

Answer: He was entitled to more for his toil. Also, he needed to pay the merchant who sold his wine (or the one who would open the barrel).

Rishonim

Rambam (Hilchos Mechirah 14:1): Beis Din must set prices and appoint officials for this, lest everyone profit as much as he wants. Rather, one may profit only a sixth for his wages, and a seller may not profit more than a sixth.

Magid Mishneh: If the price rose, the Rashbam permits to sell for the going price, even if he profits double.

Rambam (2): This refers to life essentials such as wine, oil and flour. We do not set a price for Ikarin such as costus and frankincense. One may profit as much as he wants with these.

Magid Mishneh: The Gemara did not explicitly distinguish life essentials regarding profit. It seems that the Rambam equates this to the Isur to hoard life essentials such as wine, oil and flour, but we permit spices, saffron and peppers. Also, the law itself is reasonable. I bring a proof from Bava Basra 91a. We did not resolve whether or not Rav or Shmuel permits profiting up to double with eggs, even though Shmuel permits only up to a sixth! This shows that Shmuel's law of a sixth applies only to life essentials.

Rebuttal (Kesef Mishneh): The Magid Mishneh holds that eggs are not life essentials. This is wrong. Clearly they are more essential than oil! It seems that all foods that people eat are life essentials. The Beraisa mentions wine, oil and flour for mere examples. The same applies to all foods that people eat. This is why the Rambam excludes only Ikarin such as costus and frankincense, which people do not eat. A Beraisa forbids hoarding life essentials, but permits spices, saffron and peppers. Also this connotes that all foods that people eat are forbidden. Perhaps the Rambam specified Ikarin such as costus and frankincense which are not eaten at all, but one may not profit (more than a sixth) with spices, saffron and peppers, which are used to improve food for people. Perhaps the Gemara did not need to say that Shmuel forbids two merchants to profit from eggs, because this was obvious! Also, we can answer like Tosfos says (all agree that one may profit more than a sixth if he toiled, or on eggs that he did not buy, rather, they are from his own hen - see Beis Yosef below).

Rebuttal (of both of them - Lechem Mishneh): Rav and Shmuel argue specifically about eggs to teach that they are not life-essentials!

Rambam (3): One may not profit twice with eggs. Only the first merchant may sell them for a profit. One who buys from him may sell them only for what he paid.

Rosh (5:28): One may not profit more than a sixth, unless the market price increased.

Rosh (Bava Metzia 3:16): Surely, Rav Yehudah could have found buyers who would pay a price that would enable him to profit a sixth. If not, the Gemara would not discuss his business! We learn that Shmuel's law that one may profit only a sixth refers to one who sells his merchandise in bulk without toil. For a grocer who sells a little at a time, we evaluate his toil and expenses. He may profit a sixth above this.

Nimukei Yosef (Bava Basra 46a DH Ein): One who buys from a farmer may not resell and profit more than a sixth, but if he toiled to make something new, e.g. he bought wheat and made bread, it is permitted.

Poskim

Shulchan Aruch (CM 231:20): One may profit only a sixth for life essentials such as wine, oil and flour. This refers to one who sells his merchandise in bulk without toil. For a grocer who sells a little at a time, we evaluate his toil and expenses. He may profit a sixth above this.

SMA (37): The expenses are included in principal. He may profit a sixth above the sum. We add compensation for toil only for a grocer, but not for other merchants.

Beis Yosef (DH u'Mah she'Omar ba'Meh): The Rashba says that Shmuel's law that one may profit only a sixth refers to a grocer who buys wine and Peros from a wholesaler in bulk, in order to sell a little at a time. The Gemara in Bava Metzia refutes him regarding wine. Why did the Rambam omit the Gemara in Bava Metzia?

Shulchan Aruch (ibid): This is when the price did not rise. If the price rose, he may sell like the high price. This is when there is a Beis Din to tell everyone the price to sell at. If everyone sells for what he can, he need not be the only one selling cheaply.

Beis Yosef (DH v'Chosav): The Ramah says that this is when there is a Beis Din to tell everyone the price to sell at. This is reasonable. One may derive that the Rambam (Hilchos Mechirah 14:1) agrees.

SMA (38 and Prishah 26 DH d'Chi): When there is no Beis Din and everyone else transgresses Chachamim's words not to profit more than a sixth, one who fears Hash-m need not be careful about this. If not, when he finished selling cheaply, all others will sell for a high price!

Shulchan Aruch (22): One may not profit twice with eggs. Only the first merchant may sell them for a profit. One who buys from him may sell them only for what he paid. Some say that one may profit with eggs only up to double, but not more.

Beis Yosef (DH Ein): The Tur rules like the opinion that permits to profit up to double, because eggs are not such vital life essentials. This is like the Rashbam. I say that they are more essential than oil. Tosfos agrees. He asked that one may not profit more than a sixth on any life essentials! The Rivam answered that one who toils may profit more than a sixth, but for eggs, he may not profit more than double. The Rashba says that we discuss eggs from his own hen. The limit of a sixth applies only to what one bought. The Rashbam says that there is much toil and little profit for Aniyim who seek to buy (and resell eggs), therefore, Chachamim permitted more than a sixth. This is like the Rivam.

Rebuttal (SMA 36): The Rambam and Tur hold that there are three levels. One may profit up to a sixth on primary life essentials such as wine, oil and flour. Things that are not food at all, such as costus and frankincense, one may profit as much as he wants. One may profit up to double on things that improve food, such as cumin and spices.

Drishah (28 DH v'Chosav): The Tur allows to profit like one wants with things other than wine, oil and flour. However, he allows only double for eggs. The same applies anything used for food. This shows that there is no limit only for things like costus that are not used for food at all.

Bach (28 DH Ein): Chayei Nefesh refers to things one cannot live without, i.e. wine, oil and flour. 'Borei Nefashos Rabos v'Chesronan' alludes to this. All foods give for sustenance to some extent, like we say '... Lehachayos Nefesh Kol Chai!'

SMA (39): This law applies only to eggs, for it is common that one buys from many owners and sells them. For other things, two may profit, for if the latter charges too much, one can buy from others who sell cheaply.