It would be cool if we could trade skins

The boards would be flooded with trade requests, and everyone would constantly be receiving friend invites months or years after posting something up for trade. This would also be a big hit to Riot financially; they'd probably have to remove Mystery Gifting over it at the very least.

The disadvantages are simply too huge. Just make wiser purchases instead.

So I trade my friend a skin that he already has, and he has 2 of them? Also, what it we have all the skins that we want to trade? And I will trade you my Riot Girl Tristana skin for a DJ Sona? This would completely ruin riot's income of money.