I've seen it as well, but for me, it'd be better to have premade images, and Valve can make images that interest me and aren't too cringe.

It's not a massive business, but it's probably good side money since many banks offer it. It probably costs 5 cents to print on it, and you can easily get me to give you 2$ for it. Up to 5$ if I really like it or you catch me in a moment of weakness at 2 AM.

And they can probably try to make it a big business. They managed to get people to spend stupid money on skins, levelling Steam accounts, etc, so there are probably whales who'd collect cards. For example, if you get a special card skin unlock once you 100% a game, or you just add rarity to them. The chances aren't huge IMO, but I'd have never guessed people would pay the money they pay for skins and levels anyway, so..