Gemara

88b (Mishnah): The grocer must give enough Peros to be Machri'a (lift the pan holding the weights) a Tefach (above the Peros). In a place where people measure evenly, he compensates by adding extra Peros, one part for 10 for liquids, and one for 20 for dry measures.

(Reish Lakish): The source for Hachra'ah is "Even Shelemah va'Tzedek" - give more than the full measure.

Question (Seifa): In a place where people measure evenly, he compensates by adding extra.

If the Torah requires Hachra'ah, how can he weigh evenly?

Answer: Really, the Torah does not require Hachra'ah (but where people do so, one must be Machri'a a Tefach). Reish Lakish gave the source that where people measure evenly, he compensates by adding extra.

(R. Aba bar Mamal): The Mishnah obligates a tenth of a litra (a weight, about a pound) for a sale of 10 litra (i.e. one part in 100) for liquids.

Question: The Mishnah also obligates one part for 20 for dry measures. Does it mean a 10th of a litra for a sale of 10 litra for liquids, and a 20th of a litra for a sale of 20 litra for dry goods? Or, does it mean a 10th of a litra for a sale of 10 litra for liquids, or for a sale of 20 litra for dry goods?

This question is not resolved.

89a (Beraisa): "Lo Yihyeh Lecha" teaches that we appoint people to inspect and punish for false measures, but (reasoning says,) not for prices. (If one charges too much, people will buy from others.)

The Nasi's house appointed people to supervise measures and prices. Shmuel told Karna to teach them that we appoint only for measures. Karna taught that we appoint for both. Shmuel cursed Karna.

Karna holds like R. Yitzchak¸ who says that we appoint for both, due to swindlers.

(Beraisa): If one wanted to buy a litra, half or quarter litra, we measure that amount for him.

The Chidush is that we make measures of these sizes. (We do not make measures close to each other, lest a seller use the smaller measure, and the buyer would not know that he got too little.)

(Beraisa): We do not make weights out of tin, lead or other metals. One may make them from rock or glass.

90a (Beraisa): One may keep the following (dry) measures: a Se'ah, a half Se'ah, a quarter Se'ah (one and a half Kavim), a Kav, a half Kav, a quarter Kav (a Lug), a Tuman (half a Lug), a half Tuman, and a fifth of a Lug.

He may keep the following liquid measures: a Hin, a half Hin, a third of a Hin, a quarter Hin, a Lug, a half Lug, a Revi'is (of a Lug), an eighth of a Lug, and an eighth of an eighth of a Lug (a Kortov).

We allow a Kav, even though it is two thirds of a quarter Se'ah, but not a two Kavim measure, lest it be used in place of one and a half Kavim (which is three quarters of two Kavim.

Question: Why do we allow a quarter Log and a fifth of a Lug?

Answer (Rav Papa): People can gauge small measures better.

Bava Metzia 52a (Mishnah): If a coin was disqualified and fixed to be used for weighing, it receives Tum'ah (for it has been transformed into a Kli).

Yoma 9a (Ula): Chachamim did not obligate bakers (who buy grain from Amei ha'Aretz) to separate Ma'aser Sheni and take it to Yerushalayim, because the kings' officials beat people and force them to sell cheaply.

Rishonim

Rif and Rosh (5:22): Where people measure evenly, he compensates by adding extra: a 10th of a litra for a sale of 10 litra for liquids, and one 20th of a litra for a sale of 20 litra for dry goods. R. Yochanan (our text - R. Yitzchak) taught that we appoint people to supervise measures and prices, due to swindlers.

Hagahos Ashri: For less than a litra, one need not be Machri'a a Tefach.

Nimukei Yosef (DH Aval, citing the Ro'oh): The Rif says that we enact weights twice as much as the previous, for people could err a third. A volume may be a third bigger than the previous, for people err only a quarter about volume.

Rambam (Hilchos Geneivah 8:4): We do not make weights out of tin, lead or other metals, for they rust and decrease in weight. One may make them from rock, glass, Shoham rock (onyx) and similar things.

Rambam (12): The seller must be Machri'a for the buyer when he weighs - "Eifah Shelemah va'Tzedek."

Rambam (13): He gives an extra one part in 100 for liquids, i.e. a 10th of a litra for a sale of 10 litra, and one part in 400 for dry measures, i.e. a 20th of a litra for a sale of 20 litra. This is whether he sells much or little.

Magid Mishneh: The addition for liquids is greater because some sticks to the measure, but this does not happen with dry measures.

Rambam (14): This is where people measure evenly. Where they are Machri'a, one must be Machri'a a Tefach.

Rambam (20): Beis Din must appoint officials in every province and district to go to stores and verify the scales and measures, and fix the prices. If anyone has a deficient weight or errant scale, Beis Din may hit him according to his strength and fine him as they see fit to strengthen the matter. If one jacks up prices and sells expensively, Beis Din hits him and forces him, and he sells at the market price.

Rambam (Hilchos Mechirah 14:1): Beis Din must set prices and appoint officials for this, lest everyone profit as much as he wants.

Magid Mishneh: The Rambam rules like the opinion that Beis Din appoint officials for prices because the Sugya in Bava Basra is like this, and also Yoma 9a. Also the Ri mi'Gash rules like this.

Rosh (25): R. Tam says that the Isur to make weights out of metal applies to weights for meat and oil, for the fat sticks to them, dries up and makes them heavier. One may make weights out of metal for silver, gold and all dry things, like the Mishnah brought in Bava Metzia.

Poskim

Shulchan Aruch (CM 231:10): We do not make weights out of metal, rather, from smooth rocks. Some permit to make weights for silver, gold and all dry things from metal.

SMA (15): The Rambam says that metal weights rust. If so, one may not use them even for dry goods.

Rema: This is the custom. One should cover small weights for silver and gold with leather, for if they are slightly rubbed out this causes great loss.

Source: Tosfos 89b DH v'Lo

Shulchan Aruch (14): Where people are Machri'a, one must be Machri'a a Tefach. Where people are not Machri'a, one measures evenly, and he gives an extra one part in 100 for liquids, and one part in 400 for dry measures.

Beis Yosef (DH b'Makom): Since the addition for dry measures is a Safek (the question was not settled), we are lenient for the seller, and he need not give more than one in 400. The Rashbam says that a Tefach is too big a Hachra'ah for a light matter.

SMA (21): A Tefach is only for a grocer who sells food and similar matters.

Shulchan Aruch (20): Beis Din must appoint people to supervise prices, lest everyone profit like he wants. 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. 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 Chayavim): All Poskim rule like R. Yitzchak, unlike Shmuel.

Shulchan Aruch (21): If one jacks up prices, i.e. sells for more than is proper, they may hit and punish him according to what is proper for him.