Well if talking production ready it’s not about polygons LODs blueprints or materials but more about the framework that all of this stuff can be attached to as to the need of an unlimited, only by ones imagination, number of different characters, actors, so something that is a one-off has a rather limited use.
A franchised game is usually based on unique actors that can extend out side the confines of the game. Tomb Raider would not be TR with out Lara Croft and Angry Birds would not be made into a movie, or printed on lunch boxes, if using off the shelf products where fair use would be questionable as to terms and conditions that more or less say such items can only be used in Unreal 4.
So "production ready’ would have to included an answer to whom would own the copyrights?
“Production Quality” is another story and a merchant would be best to ask how many units they have sold as to rate of return. As an end user though there are a lot of “free” alternatives as to the need of a base one-off actor that such a product as described would have to fill a unique requirement ,like buggy whips still being sold to the S&M market.
What would be of value in my world would be clothing items that fits an already established frame work.
Our project, as an example, we are making use of Genesis 3 as the frame work so stuff like hair, vests, boots shirts, pants, would be of more value over and above the G3 model which is more of a place hold.
Of interest there is already some UE4 merchants converting their products, animation mostly, so instead of being unique you stuff could be repackaged for Daz3d , UE4, Unity, or any place that sell digital assets