Question

How did the slave trade begin? Who was responsible for it?

Answer 1

Slavery has existed since at least ancient times. Are you referring to slavery at all times all over the world, or to slavery in North America? Here is a little background of slavery since 1400 from http://www.brycchancarey.com/slavery/chrono2.htm:
Before 1400: Slavery had existed in Europe from Classical times and did not disappear with the collapse of the Roman Empire. Slaves remained common in Europe throughout the early medieval period. However, slavery of the Classical type became increasingly uncommon in Northern Europe and, by the 11th and 12th centuries, had been effectively abolished in the North. Nevertheless, forms of unfree labour, such as villeinage and serfdom, persisted in the north well into the early modern period. In Southern and Eastern Europe, Classical-style slavery remained a normal part of the society and economy and trade across the Mediterranean and the Atlantic seaboard meant that African slaves began to appear in Italy, Spain, Southern France, and Portugal well before the discovery of the New World in 1492. From about the 8th century onwards, an Arab-run slave trade also flourished, with much of this activity taking place in East Africa, Arabia, and the Indian Ocean. In addition, many African societies themselves had forms of slavery, although these differed considerably, both from each other and from the European and Arabic forms. Although various forms of unfree labour were prevalent in Europe throughout its history, historians refer to 'Chattel Slavery', in which slaves are commodities to be bought and sold, rather than domestic servants or agricultural workers. Chattel Slavery is the characteristic form of slavery in the modern world...