I've some experience on the other side of the fence, seing aspiring artists trying to sell their fantasy and sf. From what I see, it's quite hard to sell WB disconnected from an actual story package, and the decisive factor is very often the actual story rather than the worldbuilding. It's very difficult to convey the interesting aspects of a setting without a driving force to move the camera, so to speak. And very easy to fall into trappings like "showcasing the world" without first establishing characters or situations.

As to how to do it, I would suggest focus on having a solid story above having an airtight setting. Then apply for young writers contests, or any kind of contest really to get your stuff read and evaluated. Once you have made some kind of "proof" of your writing ability, it's easier to get people to pay.

Other tip would be to not "waste" your big story on the first or second attempt, as it's very hard to get people to pay for sprawling and complex settings without any idea of how your stuff may work or not work (a base of comparison).