The gross error in that analogy is that there is no 3rd party involved. Each transaction have only two parties. Even if the seller have multiple clients, each transaction contains only the seller and one client.
When a person is sold, what benefits are there to that person ? The greatest loss is personal freedom. Of course, the slave will have a roof over her head and usually at least one meal a day. There is a report that a Chinese blogger advised Chinese men to look for 'youth, beauty, virginity, and fidelity' in an abducted and purchased bride. Modern day slavery and you guys here feels no shame about it. Pathetic.