it kinda depends on what types of games you guys are hoping to see on the S&box platform, imo.
I have to imagine most of the big serious games will be using custom content, so would the accessories even retain much real value outside some sort of fomo-level speculative investment (people buying hoping they go up in price later), if a lot of the popular games don't even utilize them? (I suppose it still does in roblox but idk I don't play roblox, never have)
If you look at the current charts, a good number of the games don't even use anything that can utilize the baseline cosmetics as they are, that to me is a big turn off from buying any cosmetics from you guys. I will also end up using my own assets for my games anyways most likely.
Being able to sell my own cosmetics in a separate section on my game page (shop tab when people click on game on sbox.game ?) could be nice, but obviously you'd have to vet a lot of stuff for that, but I believe roblox has that functionality baked in as well.
As for your cut, I mean it depends on if the game is f2p or paid and how much all of these servers even cost you to run. Personally, I would say make your cut more than you think it should be at first before release, and later on lower your cut if needed. That would feel a lot better than if you found out later you needed to increase your cut to help with operation costs or w/e.
Another random thought, might be cool if there was an incentive to sort of recreate some facepunch cosmetics in your games style and sync it with the customization/steaminventory stuff to display on their character in your own game, in your own way, if they have it equipped. An example would be recreating say, the hard hat as a 2d sprite for sausage survivors, and just putting it on the sprite if you detect they have that equipped on their avatar.
Having some bonus play-fund splits based on verified re-integrations of paid cosmetics (only showing if the person owns 'em) I think could be cool, makes cosmetics more valuable and sort of helps all involved with 'em.