product line vs line of merchandise

product line and line of merchandise both are nouns.

Word	Noun	Adjective	Verb	Adverb
product line	Yes	No	No	No
line of merchandise	Yes	No	No	No

As nouns, product line and line of merchandise are synonyms defined as:

product line and line of merchandise: a particular kind of product or merchandise

Other synonyms of product line include business line, line, line of business, line of products.

product line (noun)	line of merchandise (noun)
a particular kind of product or merchandise	a particular kind of product or merchandise