Bumping is done same day only at the gate. Why would an airline want to buy you off the flight beforehand when they don't know how many will show for the flight? I have seen a few instances where an aircraft was downgraded (ie: 757 to A320) which caused a flight to be overbooked a large enough number that reservations agents would call passengers on the flight and offer other flights for a voucher.

Remember, overbooking is done for a reason and I must say, the "experts" in MSP are doing a pretty good job compared to a few years ago... in LAS (one of the highest load-factor cities in the system) we probably have to bump less than 10 people on an average week... I can remember when a good day was only 10.