Steam games with a workshop are proooobably your best bet. I haven't looked deeply enough into those to know many that allow paid creations and how they handle it. I personally only know much about DotA and Warframe, and neither of those are great to mod for because 1, they both take an obscene cut of the earnings (something like 90% for DotA, and 70% for Warframe), and 2, they are both so heavily curated that there's a very large chance your creation will never even go up for sale, even if you're an extremely skilled artist.
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I personally only know much about DotA and Warframe, and neither of those are great to mod for because 1, they both take an obscene cut of the earnings (something like 90% for DotA, and 70% for Warframe), and 2, they are both so heavily curated that there's a very large chance your creation will never even go up for sale, even if you're an extremely skilled artist.