Gemara

(Mishnah): If one is Mudar Hana'ah from Yisraelim, when he buys from a Yisrael, he must pay more than the standard price. When he sells to a Yisrael, he must sell for less than the standard price;

If one vowed that Yisraelim will not benefit from himself, when he buys from (or sells to) a Yisrael, he must pay less (receive more) than the standard price;

People will not buy from or sell to him.

If one vowed not to benefit from nor give benefit to Yisrael, he can do commerce only with Nochrim.

(Shmuel): If one took a Keli from a craftsman to inspect it (to see if he wants to buy it), and it was lost or ruined b'Ones (through circumstances beyond his control), he must pay for it.

This shows that he holds that a buyer benefits from a sale (and therefore he is obligated like a borrower).

Question (Mishnah): If one is Mudar Hana'ah from Yisraelim, when he sells to a Yisrael, he must sell for less.

Inference: He may not sell for the exact value.

If the buyer benefits from a sale, this should be permitted!

Answer: The Mishnah discusses an item that is hard to sell. The seller is happy to sell it.

Question #1: If so, the one who vowed should be allowed to buy for the exact value! (The Mishnah says that he must pay more!)

Question #2 (Seifa): If he vowed that Yisraelim will not benefit from him, he must sell for a higher price.

If the item is hard to sell, he should be allowed to sell for the exact value!

Answer #1 (to question #2): The Seifa discusses an item in great demand.

Objection: If so, why must he buy for less than the value? He should be allowed to buy for the exact value!

Answer (to Question #1, and Answer #2 to Question #2): The Mishnah discusses a standard item. Shmuel discussed an item in great demand.

42b (Mishnah): If Reuven was Mudar Hana'ah from Shimon, he may not lend to or borrow from him things or money. He may not buy from or sell to him.

Question #1: Why can't Reuven lend things to Shimon?

Answer #1 (R. Yosi bar Chanina): The case is, Shimon is also Mudar Hana'ah from Reuven.

Answer #2 (Abaye): Chachamim decreed that he may not lend things, lest he come to borrow.

This also explains the other cases.

Rishonim

The Rif brings the Gemara verbatim.

Rambam (Hilchos Nedarim 7:7): If one is Mudar Hana'ah from a nation, he may buy from them for more than the standard price, and sell to them for less. If he vowed that the nation will not benefit from himself, if they consent he buys from them for less, and sells to them for more. We do not decree to forbid him to sell lest he buy (on similar terms), for he did not vow from a single person. Then, (we decree, for) if he cannot deal with this person, he deals with another. When he vowed not to benefit from a nation, he may lend to them, but may not borrow from them.

Question: Even when he vowed not to benefit from a nation, we could decree. He will benefit from other nations, like one who vowed not to benefit from or give benefit to Yisrael!

Answer (Radvaz): Chachamim did not decree because it is not common. Alternatively, we were not stringent to cause such an exertion.

Kesef Mishneh: The Rambam explains why we did not decree here like we did regarding an individual (42b, Rambam 6:16).

R. Akiva Eiger (Drush v'Chidush Nedarim 43a): The Rashba did not resolve this (why we do not decree regarding an entire nation). This shows that he rejected the Rambam's answer. I would answer like the Taz (YD 159:2). He said that we decreed (Bava Metzi'a 71a) not to lend to a Nochri on Ribis, lest one learn from his ways, but we did not decree not to buy and sell with Nochrim, for he does so also with Yisraelim. If we would decree here, he would buy and sell only with Nochrim, and he would learn from them! However, nowadays most of our business is with Nochrim, and we permit even to lend to them on Ribis, so perhaps the law of our Mishnah does not apply (and we decree to forbid all business with Yisraelim when he vowed from them).

Tosfos R. Akiva Eiger (3:11 (30)): The Gemara says that we do not decree when he vowed from all of Yisrael. What is the source to say so regarding another nation? Perhaps we do not decree only when he vowed from Yisraelim, lest he learn from Nochrim (if he may buy and sell only with them)!

Rosh (3:15): We establish our Mishnah to discuss a standard item. Both the buyer and seller benefit from the sale. If an item is in great demand, the buyer benefits. One who is Mudar Hana'ah from Yisrael may sell it for the proper price, but must pay more than the proper price when buying. If he vowed not to give benefit to Yisrael, he must sell for a higher price, but may buy for the proper price. If an item is hard to sell, the seller benefits. One who is Mudar Hana'ah from Yisrael must sell it for less than the proper price, but must buy it for the proper price. If he vowed not to give benefit to Yisrael, he may sell for the proper price, but must buy for a lower price.

Ran (31a DH u'Mocher): It is permitted to buy or sell for more or less when he forbade Hana'ah to or from Yisraelim. If he forbade the property, he may not buy or sell for any price.

Ran (31a DH Loke'ach): The Isur applies even to things bought later. Even though we hold that one cannot forbid on another something not yet in the world, here he forbids Hana'ah from a person, and the person is in the world. He is permitted to buy or sell for more or less when he forbade Hana'ah to or from Yisraelim. If he forbade the property, he may not buy or sell for any price.

Poskim

Shulchan Aruch (YD 227:1): One who is Mudar Hana'ah from Yisrael may not sell or buy with for the proper price, for both the buyer and seller benefit from a normal sale. Rather, he must pay more than the proper price when buying, and sell for a lower price. If he vowed not to give benefit to Yisrael, he can sell or buy only if he finds people who consent to pay a higher price or sell for a lower price. If an item is in great demand, only the buyer benefits. If he is Mudar Hana'ah from Yisrael, he may sell it for the proper price, but must pay extra when buying. If he vowed not to give benefit to Yisrael, he sells for a higher price, if people consent, but may buy for the proper price. If an item is hard to sell, the seller benefits. One who is Mudar Hana'ah from Yisrael must sell it for a discount, but may buy it for the proper price. If he vowed not to give benefit to Yisrael, he may sell for the proper price, and buy at a discount, if people consent.

Beis Yosef (DH Mi): Whenever an item is sold for less than the proper price, only the buyer benefits, even if it is hard to sell. Whenever an item is sold for more than the proper price, only the seller benefits, even if it is in great demand.

Taz (2): When the vow was from a single person, we decree to forbid buying in every case, lest he sell in the same situation.