Wow, so much negativity. Sounds like some photographers here haven't heard of Mpix or Instaproofs... Mpix - high quality prints on beautiful paper. I discovered them when I was prepping for my gallery show earlier this year. They will drop ship directly to your clients with no extra fee. Shipping is between $4 and $8 aprox. Instaproofs - great store front to sell your work. Yes, they do take what seems to be a "steep" 15% fee off anything you sell, but you do not have to pay to use their service and it comes with a load of great features.
My only job is photography. I make a nice amount off of prints of animal photographs. Between that and photo shoot sessions, I have enough to pay for school, a sick dog, and donate to my favorite animal shelter.
I say go for it. You have nothing to lose by trying.
I am going to go the opposite direction here and say go for it. I sell on here and on RedBubble. For clients I generally send them to RedBubble as the site is a great deal more professional than DA (not only the name itself but they aren't bombarded with annoying ads, profanity, naked sonic crap, etc) It has been rather good to me as well. Yes I could sell on my own but unless you don't want clients whom are international (most of my sales come from over seas) I wouldn't. You could however use a 3rd party like RedBubble strictly for overseas clients. The reason is cost of shipping. It is absolutely astronomical in terms of cost and drives the price up for shipping something like a 30x40 framed print to Australia. In fact it makes it downright unaffordable for anybody.
I am also realistic here, knowing that most people aren't going to drop several $100 on a print. There are of course the rare exception to the rules but realistically, most won't pay probably over $75. This is another reason why I use a 3rd party site for online orders. It also frees up my time a great deal for doing what I love, photography.
My business mode is probably quite different than most and in the coming year I plan on selling higher quality prints at art fairs and etc. For now though, all my sales online are directed towards RedBubble through my personal website.
Sorry, but this is a bad idea. Businesses that deal with any kind of artists tend to exploit and overcharge them. You can do that, but you will find in very short order that you are making very little or no profit from this kind of arrangement. You could even go into the hole (and probably will). It is best to find your own clients, make your own prints and send them to the client directly.
No service is going to sell prints to a client (at least not without a cost). They will send them to you (which you will pay for on top of printing fees), you will have to obtain shipping supplies that will not ruin prints (again, out of your pocket), and then ship to the buyer. While you can take these fees and tack them onto your prices, I wouldn't recommend it. By the time you start to actually sell anything you'll find you spend more than what you make.
Your best bet is (if you're serious about photography as a career) investing in a good printer and learning to color correct/print yourself.