Certainly they did in Africa. Most of the slaves that made it into the Atlantic slave trade (and presumably the Indian Ocean slave trade as well) were bought from Africans who sold them to the European (or Arab) slave traders.

Free black people also owned slaves in the United States, and presumably bought and sold them.

In 1830, in South Carolina, eight free blacks owned more than 30 slaves each.

In 1860, there were 10,689 free blacks living in New Orleans, and over 3000 of them were said to have owned slaves.

http://americancivilwar.com/authors/black_slaveowners.htm

In 1860 there were at least six free blacks in Louisiana who owned more than 65 slaves, the widow C. Richards, and her son P.C. Richards were the largest free black slave owners in Louisiana, owning 152 slaves, most of whom presumably worked on the sugar plantation which they owned.

In 1860, William Ellison was South Carolina's largest free black slave owner. He was born a slave, but was manumitted at age 26. By 1860, he owned one of the largest cotton gin manufacturing companies in the South, owned 900 acres of land, and 63 slaves. He was also said to have made money by breeding and selling slaves.

So, yes, it appears to be true that some black people sold slaves, both in Africa and in the United States (and presumably in other places where slavery was prevalent?)