Bank of America and Bitcoin both have market caps of ~$290 billion. Bank of America usually sees huge trading volume for a stock and it traded about $1.8 billion of volume today. BTC/USD alone has done $3 billion today.
I realize it's not a perfect comparison, but I think it highlights the fact that large sales aren't going to just decimate bitcoin's value.
Given those figures come from the completely unregulated, unaudited, incredibly sketchy exchanges themselves, I would take that with a huge grain of salt. Odds are good a vast majority of that $3 billion is trades using funny money or, for all you know, made up out of thin air. Easy to just trade with yourself all day...
I realize it's not a perfect comparison, but I think it highlights the fact that large sales aren't going to just decimate bitcoin's value.