A black market for buying and selling well trained spies. The Mercenaries Guild can sell any spies from any planet in your empire as long as they are on the same planet where the Mercenanries Guild is located.
In order to post a spy you'll need to have a Spy Pod ready on the planet. Only 1 spy may be posted per trade. Also you will not be able to train a new spy until after the arrival of the spy on his new owners planet.
Playing Tip: It is also possible to buy spies from yourself. This becomes an extra useful trick when you have the Mercenary Guild at level 30 as you can move the control on any of your spies to any of your other planets at no cost by posting and buying your own spies. This trick can even be used to build up a large army of spies on a single planet. Note spy assigned to any single planet has now been capped at 3 per level of the security ministry thus allowing a maximum of only 90 spies per planet. And since you can now train 3 spies per level of the Security Ministry this trick has now become obsolete. Before this limitation however players managed to amass armies of 10,000+ spies. But those spies all went into retirement with the new change.
You get your essentia back if the spy is withdrawn from the market.
Requires University level 10.