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. Disco ceased being a broker when the Auction House fell into disrepair.

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

Other brokers of the underworld include Pekoms and Tamago of the Big Mom Pirates.

Despite not being brokers, Eustass Kid and Killer of the Kid Pirates were seen watching Caesar Clown's broadcast about his experiment on Punk Hazard, and Kid later stated that although they have no interest in most underworld matters, they like to stay informed.