Short answers, pulled from the story.
Slavery legally ceased as an institution in the United States following the Civil War in December 1865. The United States ratified the 13th Amendment to the United States Constitution in 1865, which banned slavery and involuntary servitude except as punishment for a crime whereof the party shall have been duly convicted.
Mauritania was the last country to abolish slavery in 1981. While slavery had technically been banned by colonial France in French West Africa including Mauritania already in 1905, this had been a purely nominal ban that was not enforced in practice until legal mechanisms to prosecute those who used slaves were implemented in 2007.
According to the 1788 Census, the population of slaves in Saint-Domingue was 450,000. The census also recorded nearly 40,000 whites and 30,000 free coloureds, showing that blacks outnumbered whites by about ten to one.
The trans-Atlantic slave trade peaked in the late 18th century when the largest number of slaves were captured on raiding expeditions into the interior of West Africa. These expeditions were typically carried out by African kingdoms such as the Oyo Empire, the Ashanti Empire, the kingdom of Dahomey, and the Aro Confederacy.
As recently as the 1960s, Saudi Arabia's slave population was estimated at 300,000. Along with Yemen, the Saudis abolished slavery in 1962, though the Red Sea slave trade still provided enslaved people from Africa to the Arabian Peninsula after World War II.
At the time the weighted average global sales price of a slave was estimated to be approximately $340. The high of $1,895 was for the average trafficked sex slave, and the low of $40 to $50 was for debt bondage slaves in part of Asia and Africa.