For a rational investor it would make sense to bid above "market" price. There are many markets, and those markets have a limited volume of coin to sell. Buying 30k coins would have a big effect on the price of any single market.

Bitstamp, which says "world's leading bitcoin exchange" about itself can't sell you 30k coins right now. Oh, and bitcoin exchanges have a history of.. let's call it.. problems.

A rational investor who wants to buy a large volume of coins would certainly bid above "market" price for coins handled by a reputable seller.

You seem to have misread my post. I didn't say that no one would bid above market price, I stated that they would additionally buy up all the coins on the market below their buying price.

Wouldn't that risk driving up the market price of bitcoins, which leads to a higher competing bid on the bulk coins?

Well you wouldn't want to do it before the bids are in. However, there was a gap between when bids are placed and when the winner is announced, that would be a good time.

edit: Of course, any time after the bids were completed would be a good time. After the auction ends would be just as good for you.

Ah, you're right. But buying coin from different markets still has risks and takes time (which is money). Of course, it depends on your priorities.