In January, Sotheby’s will offer an American folk art collection with some dark and risqué imagery. The auction consignors, Petra and Stephen Levin, philanthropists based in Florida, had filled their Vermont home with woodcarvings of prostitutes wrapped around clients ($30,000 to $50,000 for two pairs) and a shoeshine boy leering at a female customer’s legs ($30,000 to $50,000). In a diorama of a bar crowded with disheveled drunks ($20,000 to $30,000), cigarette butts are smeared on the floor.