Market Control is a technology that is unique to the Trade Federation, and can be researched at the Spaceport in Tech Level 4. It costs 300 Food and 225 Nova, and removes the resource trading fee - in other words, the price for selling a resource at the Spaceport will be the same as the price for buying the resource.
Note that this will not let the player buy resources, then immediately sell them for the same price - when the player buys or sells resources, the price will change like normal. It just reduces the cost of buying, and increases the payment for selling.
"At Tech Level 4, the Trade Federation gains the ability to completely eliminate resource-trading fees."