I was in line at ORD the other day and some guy in front of me went up to the desk and asked if the plane was overbooked and if they needed volunteers. The agent said no and the guy left. I thought ok the guy was on standby but then I noticed he had a ticket already for the flight! So my best guess was that he
1) bought a fully refundable ticket
2) checked if he could get airline cash by being bumped
3) when he didn't, he returned the ticket!

Must be a loophole. I don't have time going to the airport, checking in, etc just for a "chance" of being bumped but hey some guys without jobs could make a living this way!