The slave trade refers to the buying and selling of slaves and was common in the ancient world, especially within the Roman Empire and Arabia. Although white enslavement of black Africans declined during the early Middle Ages, the slave trade along with slavery itself still goes on in Islamic and Arab countries like Mauritania and the Gulf States.
The Arab slave trade was the longest yet least discussed of the two major slave trades. It began in seventh century as Arabs and other Asians poured into northern and eastern Africa under the banner of Islam. The Arab trade of Blacks in Southeast Africa predates the European transatlantic slave trade by 700 years. 
The slave trade recommenced in 1455 in Africa. The Pope gave justification to enslave non-Christians in a papal bull. It became an important part of Africa's economy. It did not come to an end until 1800-1865 with the work of Christians; William Wilberforce and John Wesley. The United States banned the importation or export of slaves on 1 January 1808; but not the internal slave trade in the United States, which Chief Justice John Marshall and others continued to perpetrate. The Emancipation Proclamation freed slaves in the Confederate states, and the United States passed the 13th Amendment in response to end slavery in general.
I believe that the arguments for African enslavement were refined during the brutal process by Christians who needed justification but the attitudes behind the arguments were pre-existent (Drake, 1987). To emphasize, It is my position that the attitudes making African enslavement possible existed prior to the actual taking of Africans from the continent but that the refinement of the argument against Africans and for the enslavement occurred during the long history of enslavement in the Americas and Caribbean. Increasingly the Christian sentiment of the settlers became disturbed by the practice of slavery and consequently demanded new and more complex arguments to justify an un-Christian practice.
The "slave trade" was preeminently neither a trade nor an activity initiated by the victims. It was not merely a mechanism to answer the labor needs of the Americas and the Caribbean but an example of deep moral and ethical failing that relied upon the belief of white racial superiority to sustain it.
World History Lecture Eight can be used for more information.
- Foner, Eric. "Forgotten step towards freedom," New York Times. 30 December 2007.
- The Ideology of Racial Hierarchy and the Construction of the European Slave Trade