Like Peachey I am also unsure without more context. My guess is "fish markets" smell of fish, which masks all other smells. Therefore it is meant to imply that there is something wrong with whatever the person is doing which they are trying to hide.

Hi! I think that A uses the comparison to a fish market in this case to try to offend the B person. Fish markets may look messy, not so clean, with a bad smell. Also, fish markets are clearly not the correct place to buy insurances, that confirms that this is a slang used with the porpouse of criticise. The agressive tone in which B replies completes the sense of critique. This is an unfriendly dialogue.

Without any more context, I don't know. Perhaps "fish markets" refers to the people who work there, ie. the working class?