back up vs buy at

back up and buy at both are verbs.

Word	Noun	Adjective	Verb	Adverb
back up	No	No	Yes	No
buy at	No	No	Yes	No

As verbs, buy at is a hyponym of back up; that is, buy at is a word with a more specific, narrower meaning than back up:

back up (verb)	buy at (verb)
establish as valid or genuine	do one's shopping at; do business with; be a customer or client of
become or cause to become obstructed
make a copy of (a computer file) especially for storage in another place as a security copy
move backwards from a certain position
give moral or psychological support, aid, or courage to