Is a trade good for one person always good for the other?

Short answer. The statement is not correct as it stands, because of the word always. A voluntary trade between informed people is normally good for both sides, but one side can gain while the other loses when these conditions fail.

Why trade usually benefits both.

• Each side chooses to trade. A person agrees to a voluntary exchange only if she expects to value what she receives more than what she gives up. If both agree, each expects to gain.
• Exchange is not one person's win at another's cost. It is not true that one side's gain must be the other's loss: trade can make everyone better off.
• Specialisation adds to the total. When each party produces what it makes at the lower opportunity cost and they trade, total output rises, so both can gain.

Example (our own). A Nashik grape farmer sells a crate for ₹500 to a juice-shop owner. The farmer values the crate at ₹400 and the owner at ₹650, so the farmer gains ₹100 and the owner ₹150.

Why the word always is too strong. The result depends on conditions, and a trade can be good for one side but bad for the other when:

• Information is unequal. A seller who hides a defect in a used scooter gains, while the buyer, who would have refused had he known, loses.
• The trade is not truly voluntary. A person who is forced or defrauded has not chosen the exchange because it benefits her.
• Expectations turn out wrong. A buyer can regret a purchase once the outcome is known, even if the deal seemed good when it was made.

Trade can also harm people outside the deal. When a country opens up to imports, consumers gain but domestic producers of that good lose, even though the gains exceed the losses.

Conclusion. A voluntary trade between informed parties is mutually beneficial, which is why economists say trade can make everyone better off. But a trade that is good for one person is not always good for the other: this depends on the trade being voluntary and both parties being well informed.