shop assistant vs sales rep

shop assistant and sales rep both are nouns.

Word	Noun	Adjective	Verb	Adverb
shop assistant	Yes	No	No	No
sales rep	Yes	No	No	No

As nouns, sales rep is a hypernym of shop assistant; that is, sales rep is a word with a broader meaning than shop assistant:

shop assistant: a salesperson in a store
sales rep: a person employed to represent a business and to sell its merchandise (as to customers in a store or to customers who are visited)

Other hypernyms of shop assistant include sales representative, salesperson.

shop assistant (noun)	sales rep (noun)
a salesperson in a store	a person employed to represent a business and to sell its merchandise (as to customers in a store or to customers who are visited)