Europeans were encouraged not to enter Africas interior so they built ports/castles on the coast and began trade with African men in power or anyone who could get their hands on what the Europeans wanted. The main exports from Africa were Gold and Humans. When Gold was scarse would humans thrive while humans were scarse Gold would thrive vis versa
Kidnappers (slave abductors) 63%
These were native African people who made a living kidnapping vunrable people at vunrable times in order to make a living (common gangster). Their personality's were most likely lazy and greedy or in poverty or just evil. In gangs they would venture to communities at nigth and mostly take little children in the morning they could capture kids who were playing out of sight. I believe many of these people were caught in the act and themselves sold into slavery through the justice system or killed at the spot remember the general populations were against selling their peoples to the coast.
Kings, Leaders, Chiefs & their men armies 20%
These were the African men in power and most closest with the European merchants and in a sense European leaders even though the two never met face to face. European Merchants at the coast were supplying them muskets in return for Gold or humans. When these leaders were able to gather gold and sell they simply did but when it became scarce they indulged in purpose wars with neighboring kingdoms/ communities in order to have prisoners of war which could be sold to these merchants to replace the scarce gold.
Justice system 15%
In some communities most likely costly on near found it easier to sell of their criminals and gain some merchandise for the community in return for their criminal being banished from the community for good. Most criminals sold on were most likely kidnappers (slave abductors) whom got caught and sold into slavery themselves. Murderer and rapist were most likely also sold off.
Some family who found offspring out of controll or a future problem could be sold of to save hassle in future while gaining goods in return I belive these families would of had to been sick or broken and sometimes very poor. Also young women who most likley didnt want their baby could easily sell he/she on and also gain some goods in return as abortion was not an option those days anywhere on earth nor was adoption something I could think of happening during a major slave trade.
European Merchants 1%
Of course on some occasions were European merchants able to get their hands on vulnerable Africans and gain a slave free off charge. Many tactics and tricks were used but on the coastal areas and made sure not to get caught as unknown what the consequence would be. They could flash European goods at a crowd of young children prompting them to run on board their ships or into their castle where they simply kidnapped them from there but this was the least source of their slaves as I know this must of been un common.
Remember none of these groups except the Europeans and maybe the Kings didnt know where these captured people were going or what their fate was in the hands of those merchants some didnt even know they were going on ships or that Europeans were the last to have them on the continent before they ship them out it was dark times for many African regions where today it is still scared from its long lasted effects.
And you can not blame Africans themselves it was the minority who could care less about another and were greedy the majority opposed the whole trade. Only 19 out of 53 African countries had kingdoms involved in the trade.
1450-1850 : 32,000,000 captured (80,000 annual rate)
Nigeria 22% 7,040,000
Democratic Republic of Congo 20% 6,400,000
Ghana: 10.3% 3,296,000
Republic of Congo 6.3% 2,016,000
Cameroon: 5% 1,600,000
Senegal: 4% 1,280,000
Benin: 4% 1,280,000
Mozambique 3% 960,000
Guinea 2.4% 768,000
Togo 2.1% 672,000
Equatorial Guinea: 2% 640,000
Madagascar: 1.6% 512,000
Sierra Leone: 1% 3,20,000
Cote d' Ivoire: 1% 3,20,000
The Gambia: 0.8% 256,000
Liberia 0.7% 224,000
Gabon: 0.6% 192,000
Guinea-Bissau 0.2% 64,000
20,000,000 death toll (internal Africa to coastal Africa forced journey, middle passage (Atlantic ocean), torture malnutrition/hunger and disease)
12,000,000 arrive alive in the Americas