repurchase vs buy back

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

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

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

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

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

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