This sounds like something Activision decided to add rather than Blizzard. They didn't seem to be as focused on mictrotransactions until they merged, and this is the next sort of step to that.
I'm sort of wondering what kind or response this will get at Blizzcon. The item store got a lot of groans until they said, "It's for charity (mostly! partly!), but I've never heard the opening ceremony get actively booed before.
Don't we all remember the stupid ebay sales for in-game items back in Diablo II? As much as I want to be disappointed in Blizzard for allowing this, it was inevitable that they'd want their cut of the pie. What's sad is that 1) I don't see how this doesn't encourage gold/item farming in China (especially with respect to the alleged WoW slave farms) and 2) I think it was on Penny Arcade that it was pointed out that the sellers will be anonymous in the auction process - meaning that Blizzard could sell stuff directly and no one would be the wiser.
What's going to be amusing is that everything I've read has people joking that they'll be able to live off their auctions... but no one seems hyped to buy stuff. I can only hope that the bulk of the auctions are done with in-game gold and most of the real world auctions either don't sell or sell very cheap.
You believe me, don't you? Please believe what I just said...