It's a really interesting question. I suspect you'll have similar problems to me as a software developer: Getting known enough (and with a good enough reputation) to have clients come to you. As a developer, If I get work through an agency, I cannot work for the client directly for a considerable period afterwards, except through the agency.

As a software developer I have worked on a couple of Logos for people and sold them to them, I've also made logos for people as part of a website bundle. None of the websites I've produced or Logo designs have been spectacularly well paid - certainly not enough for it to be a sole income (though I've never pursued it as a main income stream).

I think the most important thing you need is: talent/contacts/publicity/website, in that order.

Good luck.

Paul