I think you’re exactly right - it is the combination of money + little oversight that is the big problem. Warframe seems to do a good job with tennogen but they limit it to only cosmetic mods and seem to be pretty restrictive about what they accept into their store. I don’t see how you could have good oversight for a game with as many mods as something like Skyrim has.
It’s not “oversight”, but if a modder needs to create their own storefront and Paypal integration, and advertising through word of mouth and their own social contacts (as in this case it seems), then that’s going to offer a lot more scrutiny than a low-effort asset flipper presenting themselves anonymously through Steam’s given storefront.