I'm not sure what you mean by 'to start slavery', but yes, Africans sold other Africans. Of couse, there was no African identity at the time, but it remains true that Africans did sell members of their own communities into the Atlantic trade.

At first the slave trade strictly followed the widespread norms of slavery in Africa - slaves were prisoners of war from other tribes and states, typically put to work on plantations, but due to the low productivity of African agriculture often more profitable when sold into the Atlantic or trans-Saharan markets. But over time, particularly from the mid-17th century on, the demand in the Americas became such that pre-existing institutions were transformed and corrupted by new elites in order to procure greater numbers of slaves. In the Kongo, for instance, the strict distinction between free Kongos and foreign captives began to break down. Whereas the 16th century Kongo monarchy protected its citizens from slavery, to the point that it would turn to diplomatic means to retrieve illegally sold Kongos from Portuguese plantations, over the course of the 17th and 18th century slave-trading elites began to undermine royal power and sell their countrymen to the Portuguese, who themselves actively encouraged political breakdown. As the socio-political order broke down, it became possible for anyone to be enslaved, even nobility, for small crimes or for simply being in the wrong place at the wrong time. For another example, in the case of the largely stateless Igbo and neighbouring people og modern Nigeria, local religious and legal institutions were transformed during the 18th century into tools of a diasporic merchant elite, the Aro, who in addition to buying up war captives and kidnapped people, procured slaves through the control of judicial oracles. So, as also happened in the Kongo, the whole legal system became a slave-trading racket, with accusations of petty 'crimes' like witchcraft and adultery being enough to have someone enslaved and sent to the coast. Similar transformations occured across Atlantic Africa, the social distance between slavers and the enslaved shrinking to the point that people were frequently selling their kin and neighbours. Atlantic African societies still haven't recovered from the culture of mistrust that developed as a result.

Sources:

Linda M. Heywood (2009), Slavery and Its Transformation in the Kingdom of Kongo: 1491-1800, The Journal of African History, Vol. 50, No. 1.

G. Ugo Nwokeji (2010), The Slave Trade and Culture in the Bight of Biafra: An African Society in the Atlantic World

Nathan Nunn and Leonard Wantchekon (2011), 'The Slave Trade and the Origins of Mistrust in Africa', American Economic Review, Vol. 101, No. 7.