Which european country first realized that there was money to be made by capturing and selling african slaves?

Portugal

Explanation:

Portuguese explorers were the first to begin the Capturing and selling of African slaves from the west coast and this was as far back as the 15th century before other European Nations joined in the exploration of Africa and capturing African slaves form the coasts of Africa.

The captured slaves were bought from African leaders along the west coast and taken away by the slave masters to their country to work on Plantations or to be sold to another Master or Another country. the Transatlantic slave Trade was one the most famous trade routes through which most African slaves were captured and sold.

b. sec, foreign relations