Final answer:
The slave sellers in the transatlantic slave trade were African tribal leaders and European merchants, with the Portuguese being the first dominant European traders before the British took precedence.
Step-by-step explanation:
The slave sellers and European 'factories' on the West African coast involved in the transatlantic slave trade were African tribal leaders and European merchants.
The African elite played a central role in capturing, transporting, and selling Africans to European slave traders, who negotiated with powerful African chiefs and established trading posts, or 'factories', along the coast. The correct answer to the question is B) African tribal leaders and European merchants. The Portuguese were especially dominant in the early years of the slave trade, but the British eventually took over as the primary carriers of enslaved Africans to the New World.