While I have never had success with using Deviantart to generate sales of my paintings, I can affirm that professionals do look there for freelance work. All of my major freelance gigs have come from art directors finding my Deviantart gallery and cold-contacting me. I am not talking about individuals, but large companies such as MTV-Nickelodeon, Baen publishing, etc.
I have found that it made my jewelry making career, as easily 80% of my sales come from people browsing Deviantart. They follow a link to my online shop and purchase there.
If you are concerned about plagiarism or copyright infringement, use a watermark on your work (you can even have deviantart put one on for you when you upload). There does not seem to be any higher percentage of stealing going on there vs. the rest of the internet. Truly, google image search has a much higher incidence of people using images in a way that breaks copyright law. Once it's online, anyone can lift it, so make it difficult for them to profit off of it. Anyone that is very determined to take something, they will anyhow, but I find that a simple watermark deters lazy thieves (these tend to be the people that will save an image, then sell the art on mousepads, etc, on eBay).
The more visible your art is on the internet, the more likely someone is to take it, but the more likely someone is to buy it, too. Food for thought!
If you are concerned about sales of existing work and paintings, you may not find it to bring you many customers. It does require that you be at least a little active, so that people can find you. As I have had almost no success in selling locally, it's worth my time sink. You have to decide if it's worth it for you, too, or not.