Slave markets and slave jails in the United States were places used for the slave trade in the United States from the founding in 1776 until the total abolition of slavery in 1865. Slave pens, also known as slave jails, were used to temporarily hold enslaved people until they were sold, or to hold fugitive slaves, and sometimes even to "board" slaves while traveling. Slave markets were any place where sellers and buyers gathered to make deals. Some of these buildings had dedicated slave jails, others were negro marts to showcase the slaves offered for sale, and still others were general auction or market houses where a wide variety of business was conducted, of which "negro trading" was just one part. The term slave depot was commonly used in New Orleans in the 1850s. 
Slave trading was often done in business clusters where many trading firms operated in close proximity. Such clusters existed on specific streets (such as Pratt Street in Baltimore, Adams Street in Memphis, or Cherry Street in Nashville), in specific neighborhoods (in the American Quarter in New Orleans, and at Shockoe Bottom in Richmond), or in settlements seemingly dedicated to serving planters seeking new agricultural laborers (such as Forks of the Road market in Natchez, Mississippi, and at Hamburg, S.C., across the river from Augusta, Georgia). Many thousands of other sales took place on the steps of county courthouses (to satisfy judgments, estates and claims), on large plantations, or anywhere else there was a slave owner who needed cash in order to settle a debt or pay off a bad bet. 
A slave market could operate without a dedicated jail, and a jail could operate without an associated market. For example, the grand hotels of New Orleans, and the Artesian Basin in Montgomery, Alabama, were important slave markets not known for their prison facilities. A number of slave jails in the Upper South were used for holding people until slave traders had enough for a shipment south, but were only rarely the site of slave sales, in part because the profit for the trader was sure to be higher in the Deep South, closer to the labor-hungry plantations of the cotton and sugar districts.
History.
Dedicated marts, depots, and lockups were by no means ubiquitous, but the slave trade itself was: "The slave trade took place in nearly every town and city in the South. In most, however, the trade did not have a permanent physical location. Commonly, slaves were sold on court days, usually outdoors at a location near the courthouse, yet those cities with a large slave market had a significant infrastructure dedicated to the buying and selling of humans." New Orleans was the great slave market of the lower Mississippi watershed—with hundreds of traders and a score of slave pens—but there were also markets and sales "at Donaldsonville, Clinton, and East Baton Rouge in Louisiana; at Natchez, Vicksburg, and Jackson in Mississippi; at every roadside tavern, county courthouse, and crossroads across the Lower South."
Slave traders traveled to farms and small towns to buy enslaved people to bring to market. Slave owners also delivered people they wanted to dispense with. Enslaved people were placed in pens to await being sold, and they could become quite crowded. In New Orleans, most sales were made between September and May. Buyers visited the slave pen and inspected enslaved people prior to the sale. People were held until their means of transportation was arranged. They were transported in groups by boat, walked to their new owners, or a combination of the two. They were moved in groups in a coffle. This meant that people were chained together with iron rings around their necks which were fastened with wooden or iron bars. Men on horseback herded the groups, or coffles, to their destination. They used dogs, guns, and whips. Railroads brought a new, simpler means of travel that did not rely on the use of coffles. 
In some cases, slave traders, like Franklin &amp; Armfield, had a network of slave depots that were located along their routes. Circa 1833, an Appalachian newspaper complained about the slave traders traveling through the region with coffles, and reported that private jails had been built by slave traders at Baltimore, Washington, Norfolk, and near Fredericksburg. According to "Nile's Weekly Register" of Baltimore in the 1840s, "The procurement of from fifty to three hundred slaves is a work of days, sometimes of weeks or months. Many plantations must be visited by the trader and his agents. Then a variety of circumstances occasions necessary delays, before the gang can be put in motion for the south. During this period the slaves are secured by handcuffs, fetters, and chains, and put into some place of confinement. The national prison at Washington city, and the state prisons, are prostituted to this use when occasion requires. The more extensive slave-dealers have private prisons constructed expressly for this purpose."
Lumpkin's Jail, the largest in the state of Virginia, was a particularly inhumane place that resulted in people dying of starvation, illness, or beating. They were so cramped that they were sometimes on top of one another. There were no toilet facilities. Swedish writer Fredrika Bremer described slave pens she saw on her travels in America as "great garrets without beds, chairs or tables." Per Frederic Bancroft, "As a rule, in all such places, the floor was the only bed, a dirty blanket was the only covering, a miscellaneous bundle the only pillow. A 1928 history described jail cells built on the Maryland farm of trader George Kephart: "...Mr. Kephart was probably the largest slavedealer in the county. He had two underground jails built where he kept the unruly, as well as a brick jail above ground." Some jails may have been tidy and officious operations, but many or most were not. Henry Bibb described one jail where he was held as repugnant "on account of the filth and dirt of the most disagreeable kind...there were bedbugs, fleas, lice and mosquitoes in abundance to contend with. At night we had to lie down on the floor in this filth. Our food was very scanty, and of the most inferior quality. No gentleman's dog would eat what we were compelled to eat or starve." St. Louis slave trader Bernard M. Lynch offered jailing services to owners for 37½ cents per slave per day.
The owners or operators of private slave jails were not necessarily the legal owners of everyone incarcerated within, and the business of jailing was distinct from the business of trading. For instance Matthew Garrison, who was both a slave trader and jail owner in Louisville, Kentucky, submitted a bill for "boarding slaves" to the county chancery court adjudicating a dispute over estate slaves, while W. H. DeJarnatt advertised that four slaves he was listing for sale could "be seen at the house of M. Garrison".
A negro mart was usually a type of urban retail market, usually consisting of a dedicated showroom and/or a workyard, a jail, and storerooms or kitchens for food. Negro marts were urban "clearinghouses" that both acquired enslaved people from more rural districts and sold people for use as farm, skilled, or domestic labor. The term "negro mart" was most commonly used in Charleston, South Carolina, but can also be found in Memphis, Tennessee, multiple locations in Georgia, et al. In the 1850s, future Confederate military leader Nathan Bedford Forrest operated a heavily advertised negro mart on Adams Street in Memphis. In January 1860, the "New York Times" reported that the Forrest &amp; Jones negro mart in Memphis had collapsed and caught fire; two people died but the bills of sale for people, "amounting in the aggregate to US$" were salvaged. A description of "the negro mart of Poindexter &amp; Little" in New Orleans, Louisiana states: "In this mart the Negroes were classified and seated on benches, as goods are arranged on shelves in a well-regulated store. The cooks, mechanics, farm-hands, house-girls, seamstresses, washwomen, barbers, and boys each had their own place." During the Civil War, Gideon J. Pillow wrote a complaint letter to the effect that U.S. Army troops had robbed him of his slaves, and killed or jailed his overseers; he wanted someone to check if the women and children, particularly, were "confined in the Ware house or Negro Mart."
It was not uncommon to hold sales or auctions outdoors in the pre-air-conditioning South; the plaza north of the Charleston Exchange may be the most enduring and notable of these locations. Similarly, rather than depending on candles, kerosene, whale oil, or gaslights, the noon-to-three trading hours of the St. Louis Hotel in New Orleans probably took advantage of the brightest hours of natural light through the rotunda windows. Outdoor slave markets were sometimes controversial. Charleston banned outdoor sales in 1856 and the traders protested that the ban might subtly send a message that there was something wrong with buying and selling people. And in 1837 a correspondent named D wrote to the "New Orleans Times-Picayune" complaining of being inconvenienced by the "practice which has been recently adopted by negro traders, I know not who, of parading their slaves for sale, on the narrow "trottoir" in front of the City Hotel, Common street...I have very frequently found much difficulty in making my way through the rank and file of men, women and children, there daily exhibited."
Many, if not most, hotels in southern cities were also de facto slave markets and slave jails. In 1884, a former slave trader named Jack Campbell told a reporter "Go into any Southern hotel that was built before the war and ask them to let you go down into the cellars. See if you don't find these old cells where the servants of travelers were shut up at night." When Reverend Thomas James, a missionary and freedman from New York, was granted permission by the U.S. Army to liberate Louisville's slave jails in February 1865 he found hundreds of people still in the pens, "many confined in leg irons," and nine more in the National Hotel.
Home and plantation jails.
Some slave owners may have had jails on their land for just their own personal slaves. A photo album of historic spots in Mississippi that was created about 1937 by the WPA Federal Writers' Project has a photo of a pleasant-looking house with a caption that reads, "Above: Sea Glen, Hancock County, Old Claiborne Plantation. It was here that J. F. H. Claiborne lived during the early 1800s and where he attempted the commercial production of sea island cotton. The old slave dungeon and the cotton press remain." Historian Orville W. Taylor described a surviving plantation jail in his "Negro Slavery in Arkansas" (1958): "A well-preserved slave jail still stands on Yellow Bayou Plantation in Chicot County, owned during slavery days by the Craig family. There are no contemporary references to the jail, but the building itself bears ample evidence of the purposes for which it was used. And it could only have been used as a plantation jail, for the nearest town, Lake Village, then a mere hamlet, is five miles away. The jail is about thirty-two feet long by twenty-four feet wide, constructed of six-inch-square rough-sawed oak timbers notched at the corners and fastened together at frequent intervals with large iron spikes. Interior partitions and ceilings are of the same construction. There are four compartments in the jail: two small cells at one end, a narrow entrance hall running the width of the building in the center, and a large cell at the other end. The interior subdivision evidently was to permit segregation of male and female slaves, and also to provide a place for the guard. Small square windows between the center hall and each of the cells permitted passage of food and water without opening the cell doors. Each cell has iron rings fastened to the walls for use in chaining prisoners. The few small exterior windows are double ironbarred, one set of bars recessed into the logs and the other bolted to the outside; the wooden-barred entrance door is also double, giving greater security. All of the hardware is made of heavy, hand-forged iron. The jail is so massive and well-constructed that breaking out of it would have been very difficult."
After slavery.
The "Smithsonian" magazine states that "[t]hese were sites of