buy back vs repurchase

buy back is not a noun while repurchase is a noun.

Word	Noun	Adjective	Verb	Adverb
buy back	No	No	Yes	No
repurchase	Yes	No	Yes	No

As verbs, buy back and repurchase are synonyms defined as:

buy back and repurchase: buy what had previously been sold, lost, or given away

buy back (noun)	repurchase (noun)
	the act of purchasing back something previously sold

buy back (verb)	repurchase (verb)
buy what had previously been sold, lost, or given away	buy what had previously been sold, lost, or given away