Another option (if you hadn't contacted Animal Control... though i think that's the smarter option...) would to be to contact the flea market directly. Let them know that you've been going there a while. That although you personally don't particularly like BYB, that the ones at the market all have clean happy pups for sale and therefore you seen no legitimate reason to complain to the market. (Most of the flea markets i have been to have sectioned with painted numbers on the ground. See if you can locate the booth number for these newcomers.) Then proceed to tell them however that this past weekend there was a new vendor with very dirty, neglected pups, that obviously have had very little personal care given to them. Tell them you find it very disturbing and have lost some amount of respect for that market in allowing those people to sell the pups there and you don't want to see them back there.

In finding a market that formally only had BYBers selling fairly well cared for pups you found a rarity. If AC does nothing, and the market doesn't value your money's opinion, perhaps you could contact a local newspaper.