Now that I think about it, I do remember seeing something about creating custom lots or something like that. The rules were different though. If you created the custom set, you would be the only one able to sell it or something like that.
I'd be curious to hear from @lawrence on this... seems to me you can just go to https://www.brickowl.com/catalog/edit, select a type of SET, then if approved you add the inventory and you're good to go! :-)
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There are 60+ entries in category “custom sets”
Someone figured it out...