What part of speech is secondhand?

Answer 1

"Secondhand" is usually an adjective: He bought a secondhand car where secondhand describes the car as used or previously owned.

"Secondhand" can also be used as an adverb: He bought the car secondhand where secondhand describes the manner in which he made the purchase.

Answer 2

"Second-hand" is either an adjective or a noun. As an adjective it means "previously used" and as a noun it means one of the hands on some clocks and watches.