TF2 had lootboxes ("crates") back in 2010, a couple of years before CS:GO was released - I think this was Valve's first major foray into them. And it was really easy to trade the items, or buy/sell them on the Steam marketplace.
I don't know how the relative market caps compared, but I remember reading years ago that a hat in TF2 had sold for $14k...
People probably not buying, but the most expensive item now on marketplace.tf is $9,499.99. Myself bought ~$200 cosmetics, mostly strange killstreak ones I play (Sheep with a gun).
But also never bought a key. Recently sold old loot boxes for $15 each, nice.
I always found it interesting that the older crates (or at least, some of them) sold for so much. I'm sure it's not unique, but the idea of selling the lootboxes themselves to other players doesn't seem like such a common thing compared to just opening them and selling the contents.
I don't know how the relative market caps compared, but I remember reading years ago that a hat in TF2 had sold for $14k...