collector's item vs rarity

collector's item and rarity both are nouns.

Word	Noun	Adjective	Verb	Adverb
collector's item	Yes	No	No	No
rarity	Yes	No	No	No

As nouns, rarity is a hypernym of collector's item; that is, rarity is a word with a broader meaning than collector's item:

collector's item: the outstanding item (the prize piece or main exhibit) in a collection
rarity: something unusual -- perhaps worthy of collecting

Other hypernyms of collector's item include curio, curiosity, oddity, oddment, peculiarity.

collector's item (noun)	rarity (noun)
the outstanding item (the prize piece or main exhibit) in a collection	something unusual -- perhaps worthy of collecting
	a rarified quality
	noteworthy scarcity