green market vs farmer's market

green market and farmer's market both are nouns.

Word	Noun	Adjective	Verb	Adverb
green market	Yes	No	No	No
farmer's market	Yes	No	No	No

As nouns, green market and farmer's market are synonyms defined as:

green market and farmer's market: an open-air marketplace for farm products

Other synonyms of green market include greenmarket.

green market (noun)	farmer's market (noun)
an open-air marketplace for farm products	an open-air marketplace for farm products