I found this in the Litré :
Marchand forain, ou, substantivement, un forain, marchand qui court les villes, les campagnes, les marchés, les foires (bien que forain signifie qui est étranger, et non qui est de la foire).
So it might be a market with a number of non-residents, non locals, what perhaps we would call "itinérant" in French a sense which is included in the very meaning of forains, somebody going from fair to fair : "foire" --> "forain". What would you say for "itinérant" ? We have also "fête foraine" with "merry-go-rounds" and the forains live in their caravans and arrive for the local annual festivity in a number of towns.