the gain that trading brings: a merchant's profit, and by extension the trading business that produces it
Sachar belongs to the world of the traveling merchant. The verb behind it, סָחַר, pictures someone going from place to place to buy and sell, and this noun names what that activity yields: the return, the takings, what the trader is left holding when the caravan or the ship comes home.
Because the business and its payoff are hard to pull apart, the word slides easily between the two. In Proverbs the weight falls on the return, as when wisdom's profit is set against the profit of silver (Proverbs 3:14). In Isaiah it falls on the enterprise, as when Tyre is called the trade of the nations (Isaiah 23:3) and the tradeable wealth of Cush is promised to Israel (Isaiah 45:14). Hebrew treats the deal and its proceeds as a single idea, so one word covers both ends.
The word is also striking for where it turns up. Proverbs borrows the language of the marketplace to price something that is not for sale, saying that the return on wisdom beats the return on precious metal, and it uses the same phrasing again of a working woman whose trading pays (Proverbs 31:18). Isaiah 23:18 takes the profit of a city famous for greed and declares that it will one day be set apart for the LORD, spent on food and good clothing for those who serve him. The word itself passes no verdict: profit is simply what trade produces, and what matters is whose hands it ends up in.