"I think he was a costermonger" said Venturney about Joe (Fleming).
From wikipedia :
"Costermongers have existed in London since at least the 16th century (...). They probably were most numerous during the Victorian Era, when they were said to be over 30.000 in 1869. They gained a fairly unsavoury reputation for their "low habits, general improvidence, love of gambling, total want of education, disregard for lawful marriage ceremonies, and their use of a peculiar slang language." (...) Even common thieves preferred to prey on shop owners rather than costers, who were inclined to dispense street justice. The costers animosity towards the police was extreme."
Here's a personal account of a costermonger, in which he discusses his selling trips from London:
"We carry them relishes from London; and they like London relishes, for we know how to set them off. I've fresh herringed a whole village near Guildford, first thing in the morning. I've drummed round Guildford too, and done well. I've waked up Kingston with herrings. I've been as welcome as anything to the soldiers in the barracks at Brentwood, and Romford, and Maidstone with my fresh herrings, for they're good customers...I always lodge at a public house in the country."
From London Labour and the London Poor, by Henry Mayhew.
I must say the way costermongers were considered adds nothing to Fleming already "unsavoury" reputation (even if Venturney was mistaken, the fact she thought he could have been a costermonger is a bit telling).