Only the big whales of 10.000 BTC would be able to have an impact on the price, and even then, it would be minimum.
We saw hundred of thousands of BTC stolen from Bitfinex and we saw a flash crash and we are already recovering. The amount of money you would need is simply too damn high.
IMHO whales holding 10.000 BTC are not fond of dumping coins because the believe bitcoins potential has not yet reached, unless the carried out a coordinated dumping to manipulate prices.
Having the number of coins is very easy to influence the market and do so competently to direct at low prices could buy more bitcoin.