I have no idea how the fine art world works, but when I saw the HN headline I thought of places that let individuals sell T-Shirts with their designs. So on that note, would it make sense to have a free-for-all section of the site, or a separate sister site, where anyone can join, then have them earn reputation points and those that become reputable there are candidates for you to invite to your fine-art section? That could bring a source of small revenue and source of cheap art to the bottom end of the market, as well as create a way to discover artists who are ready to move up.