How would a trader try to make the biggest profit from selling enslaved africans?

A slave trader would try to make the biggest profit from selling enslaved Africans by striking up business relationships with local African Chiefs, and by crowding as many Africans on boats as possible during the Middle Passage from Africa to the New World. 
Establishing a business relationships with local chiefs enabled the traders to continuously purchase new slaves, rather than trying to steal local Africans, which would lead to conflict and a lack of certainty that they would be able to continue acquiring slaves in this way. 
The conditions on the slave ships were unimaginably horrible and designed to efficiently transport as many slaves per trip as possible. Slaves were lined up in the hull of the ship, either standing or all laying down, in order to conserve space and pack as many in as possible.