Lalala:

I think Rungis is not worth the trip. No flair, commercial ambiente with hundreds of trucks - at night.
I have seen Les Halles - also at night, whole Boulevard Sebastopole closed for merchants, had a onion soup (was "invented" at Les Halles). Actually it was the red light district where the movie "Irma la Duce" played; at Rue St. Denise - the "girls" are still around.
There are many weekly markets with flair. One is in Montparnasse, near Montparnass Cementary. Another one in Quartier Latain, sorry, I forgot the name, it was near Boulevard St. Michel on a hilly, narrow, winding street.