Blacks sold their own kind into slavery
Blacks sold their own kind into slavery is a response used to justify slavery, specifically in regard to the American slave trade.
Not all black people are the same kind
Africans didn't sell their own "kind" because not all black people are the same kind. Although the USA tends to group all black people together because the slave trade effectively removed any evidence of the culture of origin of all slaves, Africa is a very large and diverse continent with thousands of different ethnic groups. There is not and has never been a monolithic black culture on the continent. To make an analogy with white people, it would be like the Germans selling Polish people into slavery and someone arguing that, since they're both white, the Germans "sold their own kind." Only someone ignorant to European history would think Germans and Poles are the "same kind," and only someone ignorant to African history would think that all Africans were of the "same kind."
It doesn't matter who sold who
Even if an African tribe did sell other members of their own tribe into slavery, does that justify slavery? Does it mean those who were enslaved deserved it? Does it absolve the people who bought and owned slaves?
It makes no difference who sells slaves; owning slaves is immoral and so is justifying slavery.