...I just looked at the Bitstamp order book: if you sell a market order of 100 BTC you will lower the BTC/USD price about 55 USD (from ~7770 to 7715). This is an 0,7% move, which I consider already significant. You can say now that most people would place a limit order, but in forex markets you would need much more volume to move even 0.1%. And if this 100 BTC market sale occured at Bitstamp, chances are that due to arbitrage on other exchanges it also would drop. With a bit more (200-300 BTC) you can already start a micro-panic. (Not considering that we're currently in a slightly bullish phase and order books are thicker than usually on the bid-side)...
Of course, you will move the BTC more easily than Euro or USD because these markets are significantly bigger. That was my major concern because from some time we see very high leveraged trades that are possible on multiple exchanges and flash crashes caused by liquidations of these trades. In my opinion, this is the main problem that causes so high volatility and stops BTC from being a safe haven.
Of course, we can do nothing to stop exchanges that offer such high leverage and we can only hope that the BTC market will grow significantly and fast without any further flash crashes which will bring the price finally to 0$. We were very close at 12.03 when BTC dropped to 4000$ and such an event is very likely to happen again because nothing changed.
Nobody learned a lesson from this event and next time there could be no such luck as last time that bots caused Bitmex to go offline for half hours during the sell-off
.