GANGNES: HUGHES AND GEDULD and STOVER both disagree with MCCONNELL about the meaning of this phrase.
From MCCONNELL 159: "The landlord fears he may be selling (not buying) a 'pig in a poke.'"
From HUGHES AND GEDULD 207: "One nineteenth-century slang meaning of 'pig' was goods or property. Hence the sentence might simply mean: 'I'm selling my bit of property.' Another slang meaning of 'pig' was nag, donkey, or moke; while 'bit of' was an adjectival term that could be used variously to express affection for the subject it preceded. ... Another possibility is a real pig, i.e., the landlord is surprised--after asking a pig buyer to pay a pound and drive the pig home himself--to be offered two pounds with a promise moreover to return the pig. According to this, people are simply talking at cross-purposes, and the narrator then explains that he wants a dogcart, not a pig."
From STOVER 98: "The landlord is puzzled by the narrator's haste to pay two pounds for his 'bit of pig' (=his valuable piece of property) coupled with a strong promise to return it."