These answers always entertain me. Most of these people are not making enough to support themselves. They make a few dollars here and there.
This is my full time job, has been for more than a decade. It's my business. I pay all my bills with it.
You have to have a plan. You have to love what you do. You have to work long hours developing websites. If you write for 3rd parties, like Hubpages your destiny is in THEIR Hands. And yes I do it... but as part of the plan. If they went away tomorrow it would be a teeny tiny bit of dropped income.
I own a whole lot of sites, I develop those that produce. If you want to be successful online you have to know a lot about marketing, web design, and most importantly you have to always put other people first! You have to test and retest.
None of what I have said is a how to. You probably won't ever make it... and that is the way it is. Because people are poor at creating plans because they do not have any idea What to do. For this reason they go to Forums, they buy a lot of products supposedly telling them what to do and how to do it - and surprise it doesn't work!
Pick one thing you know a lot about... just one thing. Then write down 10 reasons why you know enough about this to attract the attention and interests of others. Then figure out if there is a market online for what you know. See who is already promoting products and services in this market. See how many people are actively buying. Can you out compete them? Can you offer something better? Can you offer something different. Selling other people's stuff is hard because they control the customer after the initial sale.
So yes it's possible.