Slave schedules list slave owners and their slaves by age, gender and color (black and mulatto). If you know the slave owner's name, look for him on the slave schedule in 1860 and 1850. Match the owner with the approximate age and gender of your enslaved relative. I read that this is a way to find the slave owner but it hasn't worked well for me -- but you could narrow down the slaves by age to the slave owner that way.
Another method of researching slaves is to study the 1870 Census in the county where your relatives were enslaved and look for whites who have personal property and real estate. It's most likely that they were former slave owners. Also look for African Americans who lived near them, because they could be their former slaves.
Tax lists are generally not online but you can order microfilm from an Latter-day Saints Family History Center in your area. Check the LDS' website, familysearch.org to find out where the nearest Family History Center is. Also check the Locality to see what records you can order. Check the state library website or local library in those counties to see what records they have stored before you do a research trip to the area.
Do some research in those two counties to find out how slave sales were conducted. You do have the land sale record for your relative already, right? That probably has given you a lot of info.
Also if your ancestor was freed, (sorry if I'm repeating myself) it's done through either a will or a deed of emancipation, which would be a county record.
Hope this helps.