Gemara

(Mishnah - R. Yehudah): A grocer may not give treats to children, for this accustoms them to buy from him;

Chachamim permit this.

(Rava): Chachamim argue with R. Yehudah only because the grocer can say 'just like I give nuts, you can give almonds.

89a (Beraisa): "Lo Yihyeh Lecha" teaches that we appoint people to inspect and punish for false measures. We do not appoint for prices.

The Nasi's house appointed supervisors for measures and prices. Shmuel told Karna to teach them that we appoint only for measures. Karna taught that we appoint for measures and prices. Karna holds like R. Yitzchak.

(R. Yitzchak): We appoint for measures and prices, due to swindlers.

Bava Metzia 60a (Mishnah - R. Yehudah): One may not sell for less than the standard price;

Chachamim say, he should be blessed for this!

Question: What is Chachamim's reason?

Answer: This will reduce the price.

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 (Bava Metzia 33b and 4:24): Chachamim permit giving nuts because the other grocers can do the same if they want.

Nimukei Yosef (DH Gemara): When hoarders see that prices declined, they will sell for less.

Rambam (Hilchos Mechirah 18:4): A grocer may give parched grain and nuts to children or slaves, to accustom them to buy from him. One may sell for less than the standard price so more will buy from him on credit. Other merchants cannot stop him. This is not deception.

Ohr Some'ach (Milu'im after Sefer Zemanim, and Chidushav on Bava Metzia 60a): Obviously, lowering prices helps buyers. Why did the Gemara ask Chachamim's reason? Rashi answered (that we asked why he is blessed). The Rambam holds that he induces people to buy from him on credit, and through this he charges more than the market price. We permit him only because this reduces the price for people who buy from others. However, the Rambam (Hilchos Malveh 8:1) forbids charging more due to credit (this is Ribis)! We must say that credit from a store is different, for they do not set a payment date. He pays the next time he borrows. This is why we say (Shevi'is 10:1) that the first such debts are cancelled in Shemitah (for they became regular loans). No Poskim bring the Rambam's opinion.

R. Yerucham (Meisharim 9:5): The Ri mi'Gash says that we appoint people to fix prices for vendors. They may not sell for more or less.

Tosfos Yom Tov (Ma'aser Sheni 5:2): Initially, they enacted to bring (and not redeem) Kerem Reva'i to Yerushalayim from nearby places, to beautify the markets of Yerushalayim. When there were too many Peros, they enacted to redeem it outside. They stipulated that when they want, they can revert to the old law. R. Yosi says, the enactment to redeem was after the Churban. They stipulated to revert to the old law when the Mikdash will be rebuilt. Surely, the latter enactment was (not that one may, rather,) that one must redeem it, for there were too many Peros and no buyers. This is like R. Yehudah, who forbids lowering prices. R. Yosi holds like Chachamim. They did not want to beautify markets in the hands of Nochrim.

Rebuttal (Chasam Sofer Beitzah 5a DH v'Hinei): Surely all agree that one must redeem when Peros are too cheap and merchants cannot make any profit. We cry out in prayer (or blow the Shofar) due to a drop in prices of merchandise, even on Shabbos (Bava Basra 91a)! Rather, they argue about whether the enactment to redeem (and the stipulation to allow retracting this) was made when prices were low, or after the Churban.

Yad Ramah (Bava Basra 89a 132): Shmuel holds that we do not appoint people to supervise that vendors not sell for too little, for such supervision would hurt buyers. We rule like Chachamim, who bless one who sells for less! R. Yitzchak said that we appoint for measures and prices, due to swindlers. If the only concern was selling for too much, he should have said 'due to people who jack up prices!' Rather, we are concerned lest one sell for less and accustom people to buy from him because he sells cheaply, and he will deceive them through (false) measures, weights or mixing bad Peros with good Peros. In all, they will get Peros worth less than what they pay for! We conclude that we do not supervise lest vendors do not sell for too little; we do supervise that they not sell for too much, like it says in Yoma.

Poskim

Rema (CM 156:5, b'Sof): If Reuven and Shimon live in one city, and Reuven wants to lend to Nochrim for less Ribis than Shimon charges, Shimon cannot stop him. The same applies to all similar cases.

Darkei Moshe (228:3): R. Yerucham (31:6 101:4) says that if Reuven and Shimon live in one city, and Reuven wants to lend (to Nochrim) for less Ribis than Shimon charges, Shimon cannot stop him, like we say in Bava Basra regarding a grocer.

Pischei Teshuvah (8): There were two villages near each other. One wanted to sell whiskey and beer for less than the fixed price. The other village complained that this will attract its people to go there to buy. Panim Me'iros (1:78) ruled that even if the residents are Yisre'elim who will benefit from the lower price, the other vendors can stop them. In Bava Metzia, we asked why Chachamim permit selling for less than the market price, and answered that it will cause others to sell cheaply, and the price will decline. It was not obvious that Chachamim permit because it benefits buyers! This shows that we are not concerned for the benefit of buyers, since other vendors lose. The government sets a price. This is like Dina d'Malchusa Dina (the law of the kingdom is binding). Therefore, we needed to say that the price will change. This refers to selling Peros and grain. Rashi explained that when hoarders see that the price declined, they will sell for less. Selling beer (for less) will not reduce the price of grain. Surely, when the buyers are Nochrim, one village can protest. The Rema permits charging less Ribis, for there is no fixed rate for it. Anyone may charge a bit less to encourage Nochrim to borrow from him, just like a grocer may give treats to children to accustom them to buy from him. However, one may make better beer to encourage people to buy from himself. E.g. others make three measures of beer from a Se'ah of grain, and he makes two and a half measures. This is like a grocer who gives nuts, and says 'you can give almonds.'

Pischei Teshuvah (8): We may infer that the Chasam Sofer (CM 175) is even more stringent, and forbids selling to the other village even without lowering prices. A case occurred in which Reuven bought from a noble exclusive rights to sell drinks. Shimon bought such rights from another noble, and people buy from him covertly. He did not entice them; they come by themselves. Reuven lost. The Chasam Sofer ruled that if Shimon gave nuts or lowered prices to attract people, he transgressed. R. Yehudah and Chachamim would not argue about this. If Shimon did not do so, he did not transgress. If people come to buy from him, he need not dispel them. However, Reuven can protest due to Dina d'Malchusa. The first noble decreed that people under his rule may buy drinks only from Reuven. This is like his tax. One who transgresses this is stealing. Reuven is in place of the noble. When Shimon sells to villagers under the rule of the second noble, this is (Lifnei Iver,) like giving a limb from a live animal to a Nochri who could take it himself. One may protest against him, like one who divides with a thief.

Shulchan Aruch (CM 228:18): A grocer may give parched grain and nuts to children, to accustom them to buy from him. One may sell for less than the standard price so people will buy from him. Other merchants cannot stop him.