Well, it's about value... if you provide something of value then people will pay for it, if there is no value then you'll never make money from it. If you're going to sell ebooks then you'll need:
1. Specialised subject knowledge
2. A strong personal interest in the subject
3. Good writing skills
4. Knowledge of internet marketing
The second point is important... writing about something you have no interest in would be boring, if you're passionate about it then people respond to that. The first point gives what you're writing genuine value to the reader.
I've seen a lot of these 'make an ebook with our program, get rich with no work' sites around the place and they don't take into account any of the 4 points above. The forums for these sites are filled with desperate people who have lost their jobs and are hoping in vain for a better life. It's all about value, not everyone has the skills to write ebooks, and a lot of people would be better off asking themselves what they're passionate about, what value they can contribute to the market and then make money from there...