You talk about not being able to resell the cosmetics because valve doesn't allow, but many games on steam allow that, and it's up to the developer to implement the integration with steam market. It probably also gets harder to implement if the game is available in more than one storefront/platform.

You can't withdraw the money from steam, you can only spend it on other games or the market, but there are some 3rd party sites that you to sell these cosmetics and withdraw the money, but thrusting them is up to you.