Well in my opinion art assets would have a longer shelf line and constant return rate than blueprints but if the thinking is one is selling art then the returns will not be as impressive as it would be if the item was being sold as a product to be usable in someone else product. The number I would like to know is not how many units someone might be selling over time, as I doubt anyone doing serious numbers would be comfortable talking about their numbers, but how much traffic the marketplace generates every day.
Something else to consider is the marketplace is not the only game in town. Some of the stuff I see could easily be repackaged for Unity or even for Daz3D marketplace.
If one want’s to make a go of it though it’s about building up brand recognition over time and not about tossing something up that may or may not sell.