used-car vs auto

used-car and auto both are nouns.

Word	Noun	Adjective	Verb	Adverb
used-car	Yes	No	No	No
auto	Yes	No	No	No

As nouns, auto is a hypernym of used-car; that is, auto is a word with a broader meaning than used-car:

used-car: a car that has been previously owned; not a new car
auto: a motor vehicle with four wheels; usually propelled by an internal combustion engine

Other hypernyms of used-car include automobile, car, machine, motorcar.

used-car (noun)	auto (noun)
a car that has been previously owned; not a new car	a motor vehicle with four wheels; usually propelled by an internal combustion engine