How did they select the slaves during the slave trade?

Slaves were frequently captured by Black tribes as prisoners of war- they were then uloaded unto the Slave traders- who would take their human cargo to what amounted to slave markets for ( final sales). It is not often emphasized that Blacks- such as those from Dahomey- dumped prisoners of war and tribal outcasts on the Slave traders ( for profit) and thus sold their own people into the grim reality of the slave system.