buying vs purchase

buying and purchase both are nouns.

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

As nouns, purchase is a hypernym of buying; that is, purchase is a word with a broader meaning than buying:

buying (noun)	purchase (noun)
the act of buying	the acquisition of something for payment
	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

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