Cheats who travel the country, pretending to sell smuggled goods: they accost their intended dupes in a whisper. The goods they have for sale are old shop-keepers, or damaged; purchased by them of large manufactories. See Duffer.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Duck Legs * Duddering RakeNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes