How many exchanges are there that currently have 50k bitcoins for sale at a fixed price? If you bought 1k bitcoins from any exchange, it would be EXTREMELY unlikely that the orders on that exchange will still be in the same price range. I just checked Cryptsy, yes a small exchange, but there isn't even 20 bitcoins available for sale there and if all those sold, the price would be pumped to 300. Buying all those coins you're already seeing a increase is 22%. There is no way 50k bitcoins WILL sell and still keep each bitcoin under $338 a piece.
Do you also realise how many dollars you need to buy 50k bitcoins? $13.3 million USD @ $266 a piece.
So you are basically saying that the only reason one shall buy bitcoin from auctions like that is to buy large sum at once without manipulation bitcoin price? It may the case. But I find it easier to buy bitcoin from various exchanges in small packs say - less than 1k btc at once.