I mean, they could profit from a tax or fee like the Steam Market has. For example, if I sell a skin in CS where I get 100 dollars the buyer has to pay 115 dollars to get my skin. Those 15 dollars go to Steam and each time a skin is sold, Steam also makes money. I feel like this can work if Riot decides they want to make something akin to a wallet(transactions with real money) and making skin scarcity more common(limited skins as the primary skins that people can sell & trade)
Maybe these ideas of mine are dumb but it feels like a workaround to not having cases and giving a chance for people who missed out on limited edition skins to have a way to acquire & collect these skins without breaking TOS and profiting Riot at the same time.