Just as a quick example for other people who are thinking about doing this, I sell UI design & development tutorials at Design Then Code (http://designthencode.com/) for 1) designers learning how to code, and 2) coders learning how to design. I've been selling them since March and the total money earned is now about 1/3rd the salary I earn at my full-time job. Not enough to fully supplant it, but jumping my income up 33% has been pretty awesome.
Waking up in the morning to check out how much money you made last night is an incredible feeling. Or, getting an email while out at dinner that you just sold a thing that paid for your dinner? It's tough to explain just how satisfying it is. And anyone can do it. I write about the things that I do every single day. All of us have special, deep knowledge about a subject that other people might want to become more knowledgable about, it's just a matter of putting that knowledge into a compelling package.
I have the same feeling with my iOS applications. The day my son was born (a day I didn't go near a computer) I made just over $1.5k on the app store (normal sales are a small fraction of that). The feeling you get from making that much money when you didn't even think about your products that day is pretty amazing.
I think the best decision I made was finding a niche (UI development for iOS) and sticking with it, trying to build up my knowledge so that I was a very knowledgeable person in the field.
The niche worked out because it's what I'm most excited about :)
I use Quixly for payment processing, it's the service that Pictos icon set uses because Drew, the creator of Pictos, wrote it himself. It's outstanding and very cheap. I highly recommend it for selling digital goods online.
Waking up in the morning to check out how much money you made last night is an incredible feeling. Or, getting an email while out at dinner that you just sold a thing that paid for your dinner? It's tough to explain just how satisfying it is. And anyone can do it. I write about the things that I do every single day. All of us have special, deep knowledge about a subject that other people might want to become more knowledgable about, it's just a matter of putting that knowledge into a compelling package.