I understand your confusion. Firstly, kanji 「屋」 here is a suffix. It’s attached to a noun adding the meanings both “a shop where they sell it” and “somebody who sells it” So 「八百屋」 means both “a greengrocer’s shop” and “a greengrocer” 「八百」 itself doesn’t mean “vegetables”. Historically it was called 青屋（あおや）because 「青物（あおもの）」 meant “green vegetables”. It has changed the sound for some reason. It is said that 「八百」 is used because it means “large number of” and they sell many kinds of vegetables and fruits in 八百屋. When we call a person who works there, we usually add さん （八百屋さん）You could also call them as 「八百屋の おじさん」 「八百屋の おばさん」 and if they are young we might call them as 「八百屋の おにいさん」 「八百屋の おねえさん」 which is considered more polite because especially a young lady doesn't like to be called as 「おばさん」 However, we sometimes add さん to talk about the shop. Kids and women may tend to use it more often than men. It would sound soft and polite in speech. So a mother may say to her kid, 「八百屋さんに いくよ～」 "We are going to a greengrocer's shop." In writing, it would be better not to add さん to mean a shop as it might look a bit childish. Other examples: 魚屋（さかなや：fish shop; fish merchant), 本屋（ほんや：bookstore; book seller)