Kabam removed this feature for several strategic reasons aimed at protecting player progression: Roster Requirements
Kabam, the game's developer, disabled the selling option for several strategic reasons: how to sell champions on marvel contest of champions
Since you can’t manually delete them, the game handles "extra" champions through the Duplication system. When you pull a champion from a crystal that you already own, several things happen automatically: Kabam removed this feature for several strategic reasons
Selling Champions slowed down a player's progress toward "Summoner Levels" and specific titles. the game's developer
