"When I was in my early 20s, I was living in San Diego. At the end of every semester, I would Dumpster-dive at the local dorms on move-out day. I would load my truck up with stuff from UCSD, SDSU, and USD and then throw a massive garage sale. I made a killing every single time."
"I had a friend who would make $3,000 to $5,000 a year by Dumpster-diving outside of the math and engineering buildings on campus. People who wrote textbooks would send them to the department heads to be reviewed and hopefully purchased for the next school year. If teachers didn't like them, they would throw those books out, and most likely a different school would be using those exact textbooks so he'd sell them on Amazon for a fee that would benefit both him and the student."
