1
general::
Give or sell an old or used item and apply the value or proceeds to a new item. For example, Some people prefer to trade in their old car to the dealer, but we feel we'll do better by simply selling it. [First half of 1900s]
American Heritage Idioms
2
general::
Phrase(s): trade something in (for something) [and] trade something in (on something)
to return something, such as a car, to a place where cars are sold as partial payment on a new car. • I traded my old car in on a new one. • I traded in my old jalopy for a newer car.
McGrawhill's American Idioms And Phrasal Verbs