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.

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

Franky was once a trader in the underworld before becoming a pirate, with his family of thugs dismantling ships and stealing from others for selling, and Franky himself having 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 royalties also make dealings within the slave trade, despite their status in the World Government, who would also turn a blind eye to such shady dealings, showing corruption.

Crocodile, former Shichibukai, was also able to buy the illegal substance Dance Powder for his own usage, though when the government discovered, he was arrested and stripped of his status. Other brokers of the underworld include Pekoms and Tamago of the Big Mom Pirates and Eustass Kid.