Slavery

Slavery is the term used for the act of enslaving another against their will and forcing them to work for their "master".

Slavery in One Piece
200 years ago, slavery has been abolished, due to extreme cruelty. However, it still survives in certain places in the world, Sabaody Archipelago included. The World Nobles practice slavery, using wanted pirates as "pets", while other people and races as servants and trophies. To represent them as slaves, the Tenryuubito mark them with the symbol called "The Hoof of the Soaring Dragon", forever marking them as "less than human".

Unfortunately, the fact that no one can harm a Tenryuubito or stand up to them means that no one can help a slave in need, or else they would risk certain death. Aside from the "Hoof of the Soaring Dragon", the slaves also wear a Bomb Collar around their necks. If the prisoner attempts to escape from the Tenryuubito, the collar explodes, heavily injuring (if not directly killing) the slave.

Aside from the World Nobles, other people can also own slaves, if they were to buy it at certain auction houses that are (secretly) authorized by the World Government. Such example can be Tequila Wolf, where they forced many slaves to build a bridge for 700 years.

Many slaves were once infamous people. Some have bounties of more than 50,000,000 and some are just wanted by buyers for the species. Some famous or important slaves to the storyline are:
 * Keimi: a mermaid who is one of Luffy's friends, sold to Saint Charloss, but freed by Silvers Rayleigh.
 * Jean Bart: a famous pirate captain who was enslaved by Saint Charloss, freed by Trafalgar Law and joined the Heart Pirates.
 * Silvers Rayleigh: former First Mate of Gol D. Roger's crew, allowed himself to be sold in order to gain some money for his gambling debts, escaped easily.
 * Nico Robin: archeologist of the Straw Hat Pirates, building a bridge in Tequila Wolf, freed by the Revolutionaries.
 * Boa Hancock: current Shichibukai and friend of Monkey D. Luffy, slave of the World Nobles for four years, freed by Fisher Tiger.
 * Boa Sandersonia: Sister of Boa Hancock, slave of the World Nobles for four years, freed by Fisher Tiger.
 * Boa Marigold: Sister of Boa Hancock, slave of the World Nobles for four years, freed by Fisher Tiger.

Selling Slaves
The Shichibukai, Donquixote Doflamingo, owned the Auction House, before handing it over to a subordinate, Disco. To hide the fact that slavery still exists, the door to the Auction house has been labeled "Public Employment Security Office".

There is also a list of how much a slave can sell for, depending on what kind of slave the person is. The current selling price as seen within the One Piece storyline is as follows:
 * Humans: 500,000+
 * Dwarves: 700,000+
 * Minkmen: 700,000+
 * Longarms: 700,000+
 * Longlegs: 700,000+
 * Snakenecks: 700,000+
 * Fishmen: 1,000,000+
 * Giants:
 * Male Giants: 50,000,000+
 * Female Giants: 10,000,000+
 * Merfolk:
 * Female: 70,000,000+
 * Male: 1,000,000+
 * Parted Female: 10,000,000+
 * Devil Fruit Users: Market Price

Known Locations of Slavery
Sabaody Archipelago is a place where slavery is heavily enforced by sellers. Another known location is Tequila Wolf, where slaves are forced to build a bridge that connects the islands in the East Blue. It has been being built after 700 years, yet still incomplete. Also, Nico Robin is the newest slave there, after being sent there by Bartholomew Kuma.

History
It is unclear when history started, but it has been going on for more than 700 years because Tequila Wolf had been building the bridge for 700 years.

Robin told Nami that slavery was a fearful pastime of humans, reminding people at present times how fearful and cruel people were in the old days. Also, Boa Hancock and her sisters were once slaves of the World Nobles, and they suffered plenty.

Boa Hancock told Luffy the story of Fisher Tiger, who broke into Mariejois and freed hundreds of slaves from the Tenryuubito. He, being a fishman, took many fishmen and formed the Sunny Pirates, with Jinbei and Arlong included. Fisher then took the symbols of the Tenryuubito that was marked on the Fishmen and transformed them into a symbol of the Sunny Pirates, a sun.

Current Story
As seen in Nico Robin's cover story at Tequila Wolf, the Revolutionary Army arrive and liberate all slaves including Robin.

Trivia

 * Slavery was a huge trade during the real world's own Pirate Age. Some pirate crews would even recruit slaves they had freed during raids on other ships into their crews. The slaves themselves often accepted the offer as life as a pirate was often a much better lifestyle then life as a slave.