The market might exist, but I predict it won't be nearly as large as you're suggesting unless there is a departure from Unity's model. In Unity's asset store, the real money-makers are editor plugins. Given the effort put into some of the visual assets on the site, I can't imagine pure one-time sales are anywhere near profitable enough to sustain a business. A small indie shop that churns out a diverse array of content, maybe.

Perhaps Unreal will introduce some new ideas there such as a first-come, first-licensing or bidding model. Another idea might be a royalty system.