As a little experiment, here are some predictions: ArtFlock - "ArtFlock.com aims to be the foremost online destination for the sale and promotion of original art and craft by the worlds' freshest artists and makers". Fail. Avenue7 - "An online community for 12 - 17 year old girls to share ideas on the latest fashion trends & products, as well as their own personal styles". Great chance of success. I'd put all my mone…
But... I have to disagree with your artflock prediction, I think there is a market for this. A few weeks ago I was trying to buy art from local artists on craigslist for my new apartment, it was a pain though, because I had to click on each link to see the painting, I couldn't search very easily on genre or artist, and I had to wade through a bunch of postings for van gogh posters people bought from posters.com etc
Not sure if this is where they are going with that service, but a craigslist for art would be helpful to both artists and buyers.