line of merchandise vs product line

line of merchandise and product line both are nouns.

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

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

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

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

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