Isn't this any relational database where you can set the owner_id of an item?
> your entire game backend would then be built around a single marketplace that might randomly pull your game for some reason and shut down your entire company
Most online games have some sort of digital assets they have to keep track of, and that's one of the trivial things to implement.
Also, BTW: part of what makes the physical cards valuable in the long term is that even if the company that sold them goes out of business, the cards don't disappear and they still work to play the game... your dismissive attempt to build this sort of thing using centralized tech is thereby pretty ridiculous on the face of it even if you didn't try to tackle the money issues yourself.
What you want is a game that has no backend and preferably is coded using forwards-compatible technologies like HTML/JavaScript that can use these APIs to access their state for the long term... and again, that's Ethereum.
(Note: I ran a market for apps for over a decade and before that I was a game developer for almost half a decade; I thereby know what things are and aren't hard.)
I mean, if you put it like that it sounds as if the main advantage of the blockchain technology here is to offload responsibility for the inevitable issues that arise when people start trading for real money, such as scams or people regretting their purchases. But you're not actually solving these problems, you just gain the ability to point to the decentralized blockchain and claim there's nothing you can do about it.
What's the point of owning a Fortnite skin if the game is gone and you can not use or showcase it anymore? Not to mention that even if you somehow manage to retrieve the 3d asset from the information stored in the blockchain, you probably can't legally use it anywhere else due to copyright laws.