Why not? The man or woman is your property, if you can buy him or her, you can subsequently sell him or her. Buddy. If you can buy a slave, whom do you buy it from other than from someone who sells a slave? Buddy.
Read your bible, it clearly states that you can sell your slaves, although there are restrictions of the slave is hebrew (exodus 21:8). Buddy.
I really must ask by now, what do you think slavery is? I mean, you have clearly already shown that you are a sick little puppy, who thinks it is okay for one person to own another. But please enlighten me into how you think this can in any way be considered moral.