The Auctioneers

 was an organisation buying and selling alien artefacts that were found on Earth.

History
Sam Bishop was tracking them when he encountered a Code D. They had Maria Gonsalves run a research and development base in central America. (AUDIO: The Dalek Transaction)

They supplied Merx to John Torrance for his cloning experiments. Christine Colley told Kate Stewart that their capture meant that the Auctioneers would change every base they had. (AUDIO: The Sontaran Project)

On abandoning their Vienna base they left behind a travel capsule, which UNIT took possession of. (AUDIO: False Negative)

Sam Bishop found information about them and sent it to Petronella Osgood and Kate Stewart. They found some virtual reality technology and developed it for profit. (AUDIO: Game Theory)

Their main base was opposite the UNIT headquarters, the Whitehouse Gallery. Kate, Josh Carter and went through there databases to get information to defeat the Cybermen. (AUDIO: Master of Worlds)

Following the fall of the Auctioneers, other parties picked up the trade of alien material. (AUDIO: Portrait of a Lady)