Slavery has been around well since thousands of years. Farm workers, Construction, House slaves, and yes even sex slaves.
Viking Kings were know to have many female slaves besides his wife. Although vikings did prefer the British female to marry and bare his children, the sex slave industry was really booming at this time, I believe more so than now.
I am a History Chanel buff, many night I go to sleep watching. Last night the show really caught my eyes. On this very topic. A king had passed on, and a slave volunteered to be buried with him, so she would be useful in his next life..
Unlike today, at that time slaves were considered well nonhuman almost, they were beaten, raped, forced into the sex trade, and even killed for a small mistake. Killing a slave was not considered murder, but if a viking killed another mans slave, he only had to pay what the going rate was at that time.
There are also a few erotic books available on kindle you might like. Here is the link, enjoy.
Many of the slaves were chained at night to stop them from escaping and spent most of there days in shackles. They were fed at their owners convenience, not when they were hungry. They were flogged, slapped, and used for sexual gratification, without feelings.
So I suppose without the selling and buying part BDSM has really been around for thousands of years.