This has already been tried a number of times, very unsuccessfully, I'm afraid.
As moderator of an active forum for writers, media types and artists, we've bandied this idea about quite a bit and the IDEA sounds great. In reality, when you start asking people to ante up the money to pay, they're full of "excuses."
Our metrotribnews listserv is open to writers, visual artists and performers around the world. Again, the reality is that writers tend to want to be where writers are, and photographers, with photographers, performers with performers,e tc. I can't see that any one web site or service could do a great job at servicing all the unique needs of such a varied community OTHER than as we do, to share ideas and collaborate.
Finally, and I think this is most important....you really do not know WHO is handling your work when you are dealing, online. If services are available, it will be absolutely necessary to establish the credibility of the staff handling the material of your client-artists. A person who is good at marketing painters will not necessarily be as great with handling writers.
The Metropolitan Tribe: artists making connections, sharing resources & forging success! http://www.metrotribe.org
Join The metrotribenews & connect with artists & writers around the world: http://groups.yahoo.com/group/metrotribenews/