Smithfield Bargain: A bargain whereby the purchaser is taken in. This is likewise frequently used to express matches or marriages contracted solely on the score of interest, on one or both sides, where the fair sex are bought and sold like cattle in Smithfield.
-- A Classical Dictionary of the Vulgar Tongue by Francis Grose, 1788.
'Smithfield Bargain' was once a common enough phrase, but a confusing one. Smithfield market, London1 has a long history as a livestock market, and at least among the upper classes, was disdained as a particularly poor place to buy a horse. A 'Smithfield jade' was any inferior horse which had been mangonized up to deceive prospective buyers. It also came to be used as an euphemism for a prostitute.
Meanwhile, London's divorce laws were non-existent (prior to 1857) or levied an exorbitant fee (after 1875). Spouses wishing to change mates did so, and often were cause for gossip; for example, in 1876 a man agreed that his wife could move in with her longtime lover and become his "wife"; they closed the deal with the usurper buying the cast-off husband a gallon of beer, and another pint upon receiving permission to additionally adopt the child from the first marriage. This scandalous -- and illegal -- behavior amongst the lower classes received a write-up in the The Sheffield Daily Telegraph. While reports of such goings-on were not especially rare, it is important to remember that they were reported because they were interesting, not because they were common.
However, while Smithfield market had a repetition for having this sort of thing happen, it is unclear not only how common such things were, but also if they were particularly common in Smithfield, if perhaps some of the Smithfield bargains were actually cases of prostitution2 rather than buying wives, and if, perhaps, the association with Smithfield market was primarily because of the association of a cattle market, rather than any events tied to Smithfield market itself. Comic illustrations of such goings-on loved to use the imagery of people as livestock, or women wearing halters; Smithfield market fit right in with this metaphor.
All of which is really a side-note. We have little remaining evidence of how this phrase may have been used in the streets, with nearly all of the existing examples refering to something quite different; an entirely metaphorical euphemism for marrying for money or an advancement in social class, or an arranged marriage (also, generally, for money).
Of course, written usages were, at the time, written by the upper classes, so it shouldn't be surprising that they used it in a classier sense. 'Smithfield bargain' was a disparaging way of refering to otherwise respectable people engaging in the time-honored tradition of putting wealth and status before love.
While Francis Grose should probably be trusted as a good source on the way the commoners used the term, it should also be noted that the 1807 edition of The Complete Farmer uses the term in the sense of an uncertain deal,3 but not a bad one. Finally, it should be noted that the use of the term by the upper classes would have carried with it a sideways reference to a Shakespearean quote, which no doubt explains some of its popularity:
Falstaff: "Where’s Bardolph?"
Page: "He’s gone into Smithfield to buy your worship a horse."
Falstaff: I bought him in Paul's, and he’ll buy me a horse in Smithfield: an I could get me but a wife in the stews, I were manned, horsed, and wived.
-- Henry IV, Part 2, Act 1, Scene 2, William Shakespeare, c. 1597.
Footnotes:
1. There was, and is, also a Smithfield horse market in Dublin. This is probably not related.
2. It should be noted that the waters are further muddied by the fact that some moralizing Christians were quite happy to refer to stable, long-term relationships not formalized by the church as 'prostitution'.
3. A link to the passage in Google books.