"Ninety-nine percent of the $14.6 billion raised by nonprofits in auction in 2005 was raised offline." (Forbes, August 14, 2006, P. 105)
I love that sentence. 99%. $14.6 Billion. Offline. That's data for you.
Of course, Forbes (the Magazine - old company) got the data from Cmarket and BenefitEvents.com (web-based technology firms - new companies).
The biggest online auctioneer - Ebay - has a Giving Works program that also supports online auctions for nonprofits.