While you cannot sell a champion for Gold, you can "cash in" duplicate champions.
There is one specific exception regarding "Selling." If you purchase the Sigil (the monthly subscription) , you gain access to the "Bunker" store. Here, you can purchase "Sigil Stones" using Battle Chips. However, you are still not selling your champions; you are buying items to increase your champion's power. how to sell champions on marvel contest of champions
The “Sell” button in MCOC is a trap for new players and a tool for micromanagers. While you cannot sell a champion for Gold,
Kabam, the game's developer, disabled the selling option for several strategic reasons: the game's developer