piece goods vs merchandise

piece goods and merchandise both are nouns.

Word	Noun	Adjective	Verb	Adverb
piece goods	Yes	No	No	No
merchandise	Yes	No	Yes	No

As nouns, merchandise is a hypernym of piece goods; that is, merchandise is a word with a broader meaning than piece goods:

Other hypernyms of piece goods include product, ware.

piece goods (noun)	merchandise (noun)
merchandise in the form of fabrics sold by the yard	items for sale to the individual consumer
	branded products meant to promote another product, especially films and pop groups

piece goods (verb)	merchandise (verb)
	engage in the trade of