You can sell your ebooks through Amazon's Kindle program or Apple's iTunes Connect, which gives you access to a large majority of the digital-book reading market and the barriers for entry are incredibly low. You don't need to invest lots of money to make this happen, but you do need to invest lots of time, not only in writing the ebooks, but in marketing them as well.
It is likely that at one point or another in your life, you have needed advice to help make certain decisions. The advice provided is usually from someone we trust or an individual that has gone through a similar experience making them an expert on the matter. It is for this reason that online companies that offer advice through chat have been set up to make it easier for online users to have their questions answered.
Hack for hire is possible, selling their 'hacking' skills as a service. However this will only work for a limited minority with the right connections as well as well as the right skills. Discovering and selling exploits and root kits is possible, selling pirate software, music and DVDs. There are grey activities such or bounty hunting exploits or Botnets for game gold farming, against T&C but not criminal law.