There are a bunch of sites out there, not just microstock that accepts eps files only (?).
In
this topic, society6 was mentioned.
Reading their description, they are fine with exported rgb images.
http://society6.com/help/prints There are other spec work sites like 99designs where there are many different "contests", that can be about various things, even 3D models.
My personal experience is very bad with it though, it pays me less than 1% of my time wasted on it.
I would also love to get suggestions from a freelance designer who can make a living out of it.