For the unawares: Steam publishers can add items to their games which can be traded on the Steam Marketplace for pennies, or if it’s rare enough then you can just set the price yourself because nobody else can sell it cheaper. You can see the sale history to know if anybody is actually buying or selling the item.
This lead to a developer making a game where you click a banana and every 8 hours or so you can earn a marketable banana of varying rarity. He never actually implemented the items ingame, he just kept adding more collectible bananas.
Now that but also cats, I guess.
There are actually some steam marketplace items that were bought and sold for exceptionally high prices, such as rare cosmetic couriers in Dota 2, or rare hats in Team Fortress 2. I remember when some Golden Immortal reskins for Dota 2 were bundled with a physical figurine purchased at one of the events, such as the Golden Empyrean staff and ultimate ability cosmetic override, which just recently sold on the market again today for $330 USD and that’s not even a very coveted item, tbh.