I'm familiar with selling through etsy, but not art so I'm not sure how much use I can be. Setting up a shop requires a paypal account and is really quite easy, but will take a bit of time making sure policies, shipping and so forth are correct. They charge $0.40 per listing for 4 months, and paypal takes a small percentage of income as their fee. You also have to be sure you can ship whatever you sell for a reasonable amount. It helps to have as many items as possible (it kind of gains critical mass) in an etsy shop, preferably at a few price points and with staggered renewal dates so a broader range of search criteria will bring you up. The more targeted or unique you can be, the better as well. Can you make cards, tags, or other items for weddings or other occasions? At a certain point it can get pretty elaborate and if this is a side thing you should probably figure out what your limits are beforehand. Oh, and learn how to take really great photos of your stuff (the print itself- not just the jpg file), and make sure the thumbnail is eye catching- #1 key and probably the most time consuming set-up part.
In regards to prints, I know of several people who sell through Society6, but I'm not familiar with it myself.
I have the feeling in regards to art, that these places will tend to be more like a sale point to which you will have to provide advertising (whether online or real world) to drive customers. Deviantart, Tumblr, a blog and so forth are all online options, but do your own research and find places that are most relevant to your type of work.
This may be an aside in some ways, but if you're level quality is high enough, I would also look into getting into a gallery or store when possible. There are often places which will make an effort to find local unique work. Yes they take a big cut, however they can provide a higher end setting which makes your work look even better, and can often get clientele who have deeper pockets and are already looking for what you do. It's also a lot less work day to day. YMMV