i just checked 3 relatively legitimate bitcoin exchanges (kraken, gdax, bitstamp). you could put a 300k sell order on any of them and have less than 0.5% of slippage, which is much less than regular trading volatility. plus if you really wanted to, you could spread your trades among multiple exchanges and execute them simultaneously for even lower slippage.
you don't. how most exchanges work is that when selling, you can place a limit order (sell at that price or higher), or a market order (sell at whatever price). if you want certainty, you use limit. if you want to make a trade now, you use market.
Ah, so that 0.5% was just a wild guess. Others have chimed in that they've sold a bunch in one day and been fine, but ... there's always folks that want to pump the price, so I'm not sure what to believe.