trade in something—buy and sell a particular commodity:
- The company of merchants included both general dealers and such as traded in only one kind of wares.
Note: The expression does not correlate in meaning with the phrase trade something in—give a used thing as part payment for a new one:
- You trade in your car and the minute you get used to the thing some bastard puts out a new model.
trade on something—take an unfair advantage of a person’s weakness, ignorance, etc.:
- Employment agencies have been characterized as a class who trade on the needs of the inexperienced searcher.