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
Though the practice was prohibited by the World Government 200 years ago in the current storyline, slavery 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.

Bomb Collar
The Bomb Collar is a device strapped around the neck of slaves. They are connected to a chain, which, if broken, will cause the collar to explode. Their purpose is to prevent slaves from attempting to escape, and only the key or a highly skilled person is able to unlock it. It first appeared in Chapter 497 and Episode 391, the collar being on Dias.

Noteworthy Slaves

 * 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.
 * Koala: merely a young girl who was forced into slavery, it being so tramatizing to her even after being free for three years she continued to act as one.
 * Fisher Tiger known as the Adventurer, was a fishman and the founder and original leader of the Sun Pirates who freed all the slaves in Mariejois.

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 series 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 slavery started, but it has been going on for more than seven hundred years because the people of Tequila Wolf had been building the bridge for seven hundred 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 Sun Pirates, with Jinbe 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 Sun 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.

Anime and Manga Influences
Slavery was a huge trade during the real world's own Pirate Age. The slaves were often of African origins and had travelled across sea in appauling conditions with many dying along the way. The ones that survived the journey often went onto live a life of misery and abuse, sometimes in conditions not much better then the ones they travelled across the sea in. The branding of slaves with the owners mark was practice in the real world. Just as in the One Piece storyline, in many cases were no longer considered human beings.

Those slaves who ended up being sold often tried to escape; slave owners over time turned to putting traps around the edge of their property to prevent the slaves leaving, much as the exploding collar on the One Piece slaves serves this same purpose. The slaves lucky enough to escape were unable to return home and escaping slavery was also a risky situation; when caught by the authourities, if they were not returned to their masters they may have even have been put to death for escaping their master. Those that were returned to their masters often faced serious punishment, usually by beating, at times the punishment was so brutal that it may have even led to the slaves death. In the One Piece storyline, such a thing was seen with the Sunny Pirates, where the Marines attacked Tiger Fisher demanding that he return the slaves he stole, even though it was impossible to tell who was a slave anymore.

As pirate crews raided ships at sea, it was not uncommon for these crews to come across slave ships. Some crews gave the slaves a chance to join the crew, these slaves often took up the offer as life as a pirate was more often then not a better lifestyle then life as a slave. The slaves were often very thankful to their freedom and very loyal to the crews they joined. The recruitment of slaves into a pirate crew lso led to a number of crews being multi-racial. #In iOne Piece, Tiger Fischer himself was not against the idea of taking Koala on board his ship or returning her to her home island and even burnt the "mark of the sun" onto her back, despite being a human. The crew itself consisted mostly of Fishmen, but at least one Merman was also known to be on board with the crew.

Trivia

 * One could argue the exploding collars the slaves wear are a reference to The Running Man. In the storyline, prisoners held within a prison had to wear collars that would explode if they attempt to leave, just as the slaves collars explode if they attempt to escape their masters.
 * Though she was not sold, Victoria Cindry was a slave to Hogback. She obeyed his every demand and never talked back, though she did say rather negative things to Hogback.