Why doesn't Riot let us resell champions for half their blue essence price, or some similar system?

There are a lot of champions I would love to get rid of! I unlocked those champs because of hype and then later decided I hate. Its inconvenient not only for ARAM, but it clogs up my roster. The only reason they are of any use to me is that I can "trade them" to other players in champion select so they can get the champion they want.

I don't even care if Riot deleted all the skins for that champ when they were sold. In theory that could make money for them even... if they decide to buff a champion someone had sold, that player would not only need to unlock the champ again, but buy any relevant skins again.

If this comes out as a win for Riot, why haven't they done something like this? I genuinely want to know the reason.