According to the 1830 census there were approx 319,599 free black slaves about 14% of the black population. A significant number of these free blacks were the owners of slaves, the census lists 3,775 free blacks who owned 12,760 slaves. Of course almost all the slaves transported to the new world were bought off black slave owners in Africa. Native Americans also owned thousands of slaves. Slavery was world wide and the Islamic African slave trade far larger, but it's only thought of as a transatlantic crime. I recommend you read some Thomas Sowell works.