I am convinced that DL does use the bid system, but not for the purposes for which it is said to be used. DL clearly doesn't use the bid system flight-by-flight at the gate. But, it does collect a great deal of data about what people will accept and by linking that to the bidders' travel patterns.

Overbooking is a statistical game based on prior behaviors. If DL knows that on a given flight it can "buy" VDB's on the cheap, it will overbook by more. Sometimes the system will be wrong and DL will have to pay out more. But, more often than not, it works.

If DL could regularly get volunteers on TCONS for $50, that's what the VDB offer would be.