Broker

A broker (仲買人（ブローカー）) is an individual or a party who makes a transaction or dealing with others.

Brokers in One Piece
In the series, the brokers focused on are underworld dealers, trading in illegal goods such as slaves of different species, highly priced goods, or weapons of mass destruction.

Known Brokers
A very famous broker is Donquixote Doflamingo, who operates under the epithet "Joker" in the underworld. He once possessed the Human Auctioning House, where slave trading thrived. He is also known to be collaborating with Caesar Clown in dealing chemical weapons of mass destruction, among other business arrangements. In his dealings, he also has had subordinates such as Disco to help him to make the transactions.

Crocodile, the former Shichibukai, was able to purchase the prohibited substance Dance Powder for his own use, though when the government discovered this, he was arrested and his status was revoked.

Franky was once a merchant in the underworld before becoming a pirate, with his family of thugs dismantling ships and stealing from others for selling, and Franky himself had connections in the black market, enough to acquire the immensely rare wood from the Treasure Tree Adam.

The World Nobles, along with various Nobles and personages also engage in commerce within the slave trade, despite their elite status within the World Government, who would also turn a blind eye to the underworld market, demonstrating further corruption.

Other brokers of the underworld include Pekoms and Tamago of the Big Mom Pirates. Eustass Kid and Killer of the Kid Pirates also have been shown to be members of the Broker Underworld. However, they had no interest during Caesar Clown's broadcast but wanted to stay well informed.