MimiTaisho
In this area you can create a cage for your slave and await a master or mistress to come pick them up.
If you wish, you can also mention that the slave could be auctioned or bought in several different ways instead of from a simple cage. like a slave walking around trying to get the attention of masters so they could be bought (like a black market slave trade). as well, people might want to post the link to their slave or owner page so the slave they are interested in, or the owner that slaves might be bought by, can easily see what kind of character/person they are.