Why hasn't Riot added a skin shard trade shop

A Skin shard trade shop would be very good for them people would buy more chest meaning more money spent in the game meaning more money for Riot it would not even be complicated for example Pulsefire Ezreal 3250: Wants Elementalist Lux 3250:

Or something like this

Omega Squad Tristana 1350: Wants Warrior Princess Sivir 520 Crimson Elite Riven 520

And then there would be 310 RP left that can be given to you in Orange Essence

This is just what I thought could be possible in the future I doubt it will happen though