second hand vs secondhand

second hand is a noun but secondhand is not a noun.

Word	Noun	Adjective	Verb	Adverb
second hand	Yes	No	No	Yes
secondhand	No	Yes	No	Yes

second hand (noun)	secondhand (noun)
hand marking seconds on a timepiece
an intermediate person; used in the phrase `at second hand'

second hand (adjective)	secondhand (adjective)
	previously used or owned by another
	derived from what is primary or original; not firsthand

second hand (adverb)	secondhand (adverb)
from a source of previously owned goods	by indirect means