Hi Marcscime, thanks for a good question :) As you might know the market leader Turbosquid has around USD 35mn in revenue per year, so a good indication of the overall market for paid models would probably be doubling or tripling that amount. The demand for free models is much, much higher - but obviously hard to quantify in monetary terms.
In our marketplace buyers come from a variety of backgrounds, including architecture, advertising, scientific / medicine fields, gaming, visualization, and more recently 3D printing. The clients tend to be both freelancers and companies on a deadline, looking for something to meet the deadline. If the buyer is a time-sensitive corporate, the most important aspects of a model are whether it suits their need (you won't buy a flower if you need a car), and the quality (so that you do not spend a lot of time on fixing the models). If the buyer is a student buying for own personal project, the most important aspect is of course the price.
Hope this answers your questions at least a bit - let me know if you have any further ones!