The middlemen involved were;
Arab Swahili traders
Ways of obtaining slaves
Selling of domestic slaves in exchange for goods like beads, guns, glass etc
Selling of criminals, debtors and social misfits in society by the local chiefs to the Arab slave traders.
Prisoners of war could be sold off.
Porters were sometimes kidnapped, transported and sold off to the Arab traders.
Raiding villages, this would begin at night with gun shoots and people would scatter consequently leading to their capture.
Through inter tribal wars many Africans become destitutes and these would be captured by the slave traders.
Tax offenders were sold off by the African chiefs.
They were also captured through ambushes during hunting, travelling and gardening.
Slaves would be acquired from the main slave trade market in Zanzibar.
Other Africans are also said to have gone voluntarily in anticipation of great wonders and benefits from the Arab Swahili traders.