It would be nice to have an "adult" checkbox for marketplace submissions. Any such items could for the time being not be listed on the marketplace at all, but still be sold from domains with people who get the direct link. That way creators can take advantage of the current tools and processes in place of the marketplace without going against the current submission policy for no adult content.
I assume there will be people who want to see this. I certainly do. Again this would depend on management I guess, vision and timing, hence why the suggestion is to have the items not listed on the marketplace at all instead of having them on a separate adult listing, which could be a secondary step in the future.