Official statement from Valve.
We shared with the NYAG that these types of boxes in our games are widely used, not just in video games but in the tangible world as well, where generations have grown up opening baseball card packs and blind boxes and bags, and then trading and selling the items they receive.
You’re right! We should stop that too!
While you can generally also earn them through gameplay, it’s normal for lootboxes to be available for sale, either directly with money or via an intermediate currency that can be bought with money (which is generally specifically to skirt anti-gambling law or to force you to buy an amount of the intermediate currency that doesn’t exactly match anything you can buy with it).