OP got taken by Delta. Yes sure he agreed to it, but they violated the spirit behind VDB and why airlines overbook in the first place.
They know that most of the time they can sell more tickets than seats and the math will work out for them. And because it works so well 99% of the time, the 1% it's worth paying people to volunteer to take a different flight. And way cheaper then IDBing people.
This 5 minute TED talk shows the nitty gritty math.
https://www.youtube.com/watch?v=ZFNs...ature=youtu.be.
The key questions are to settle payment, route, and accommodations if it is overnight. Before agreeing. There will be pressure from the GA as they are trying to get the flight out on time and have to move on to other PAX if you end up not agreeing.
Rarely will someone be willing to do it for free, especially since almost everyone knows airlines start at 200+ to get things moving. People don't like uncertainty and changing flights creates uncertainty.