purchase vs buying

purchase and buying both are nouns.

Word	Noun	Adjective	Verb	Adverb
purchase	Yes	No	Yes	No
buying	Yes	No	No	No

As nouns, buying is a hyponym of purchase; that is, buying is a word with a more specific, narrower meaning than purchase:

Other hyponyms of purchase include buyback, redemption, repurchase, purchasing.

purchase (noun)	buying (noun)
the acquisition of something for payment	the act of buying
the mechanical advantage gained by being in a position to use a lever
a means of exerting influence or gaining advantage
something acquired by purchase

purchase (verb)	buying (verb)
obtain by purchase; acquire by means of a financial transaction