What aspects of buying and selling of slaves does northup single out for condemnation?

The degrading show and presentation of slaves--Soloman Northrup's description of the slave auction shows his disgust for the examination of slaves as if they were horses on parade. The act takes away humanity and creates property. 

Northrup describes slaves having to open their mouths to be inspected. Buyers would inspect the bodies of slaves looking for marks or physical attributes they needed in a slave. The process includes cleaning up slaves to help improve the value. 

Hoped he would accept their demands and end the violence