Sham leggers; cheats who pretend to sell smuggled goods, but in reality only deal in old shop-keepers or damaged goods.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Leg * Lenten FareNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes