Microsoft getting their hands on anything this big can't be a good thing.
Personally, I think they have enough on their plates trying to make a stable OS... they don't need to worry about billions of auctions.
Besides... if microsoft bought ebay... we'd end up with the blue screen of auction death, about 1 minute 30 seconds from the end of a high end auction with a bidding war going on... all the bids would be lost and we'd be charged double the fees for our troubles, with no refund...