Not always the case. Call of Duty > $1 billion market cap > You can get 20% of that market with a website. Seems easy, but it takes hard work, and brains. Oh, and timing.
My favorite niche is writing ebooks and selling them for the kindle and nook. I don't write SEO or IM books though mainly because I'm still learning. I also like blogging but I am still figuring out how to make good money with that.
Everyone has their own niches but most successful people seem to find health, nutrition and making money are the 3 best niches.
the best site to earn money atm has to be https://www.adcashblast.com/?00001305 not only do you earn money but you get free advertising also.