The custom of servitude, thus commenced, is soon fed by new sources.

Prisoners of war are enslaved, or, at the will of the victor, exchanged as an article of commerce.

Before the interchange of money, we have numerous instances of the barter of prisoners for food and arms.

And as money became the medium of trade, so slaves became a regular article of sale and purchase.

Hence the origin of the slave-market.
Luxury increasing slaves were purchased not merely for the purposes of labour, but of pleasure.