Before the Civil War, the market for slaves consisted largely of people engaged in plantation agriculture. These buyers made their money through the cultivation and sale of cash crops such as tobacco, rice, cotton, and in some places sugar or indigo. The further one gets into the 19th century, slaves became more commonly used in grain agriculture, as the soil quality in parts of Maryland and Virginia had deteriorated to the point that tobacco had become less profitable. Plantations are not the only place you would have found slave labor. Small farmers owned slaves in much smaller numbers to perform a variety of tasks, and slaves labor was used in the South's growing industrial sector. There was also a market for slave labor in cities, and renting surplus slaves to others was not an uncommon practice either in the cities or in the countryside. It is important to remember that slave labor was present in the North as well, but it was abolished there earlier, and with considerably less bloodshed. Depending on what part of the North you look at slavery took varying lengths of time to disappear. States such as New York, for example, passed laws abolishing slavery only gradually, and the practice was not outlawed in Connecticut until 1848. In New Jersey, there were still enslaved people living in the state at the outbreak of the Civil War.

During the 19th century, the slave trade was mostly internal after Congress outlawed the importation of slaves into the United States from overseas in 1807. In practice, slave importation had been made more difficult by a series of statutes starting in the 1790s that made direct legal participation by US nationals in the international slave trade harder. Illegal slave trading expeditions, however, did continue through the 19th century. Even the constitution of the Confederate States of America, it should be noted, prohibited the importation of slaves from overseas. During the 19th century the market for slaves consisted largely, but not exclusively, of those participating in plantation agriculture. In the antebellum era, this was commonly associated with the burgeoning cotton trade, which exploded in the southeastern US in the first half of the 19th century. The growing importance of cotton thus drove much of the demand for slaves, and many slaves on the cotton plantations in states such as Alabama and Mississippi were brought in from states such as Maryland and Virginia. This transfer of slaves from one part of the country to the other formed an important part of the South's economy, and is sometimes called the Second Middle Passage. When we talk about the slave trade in the 1800s, and by the time we get to the Civil War, we are basically talking about a domestic, internal trade. The trade was so important that important aspects of the South's finance sector grew up around it, including a 19th century form of consumer financing.

So yes, the importance of slavery in the South can largely be attributed to its role in the production of cotton, though it did have a longer history that preceded that.

Readings

David Brion Davis, Inhuman Bondage: The Rise and Fall of Slavery in the New World

James Oakes, The Ruling Race: A History of American Slaveholders

Ira Berlin, Many Thousands Gone: The First Two Centuries of Slavery in North America

Ira Berlin, Generations of Captivity: A History of African-American Slaves

Steven Deyle, Carry Me Back: The Domestic Slave Trade in American Life

Calvin Schermerhorn, The Business of Slavery and the Rise of American Capitalism, 1815 - 1860

Paul Finkelman, Slavery and the Founders: Race and Liberty in the Age of Jefferson

Don Fehrenbacher, The Slaveholding Republic: An Account of the United State's Government's Relations to Slavery

Walter Johnson, Soul by Soul: Inside the Antebellum Slave Market