I think you answered your own question. Any side business is possible. It depends on what you like or want. Some people do good things while others do bad. Unfortunately, some struggling traders who had failed systems joined the ranks of the infomercial business and started selling basic knowledge at a premium price. People would pay thousands of dollars for a seminar or course that offered basic info from a rehashed novice's book. I think these individuals were more desperate to make money than spend the time to experiment and educate themselves.
Buying/Building and then selling aircraft.... http://www.epicaircraft.com/LT.html * Aircraft in several segments are STILL appreciating!