The normal trading volume isn't really the key. Rather, it's how elastic the price is.

Suppose these guys sell 10% of the daily trading volume. How do the traders in the market react? One possibility: Buy at current prices. Another: Speculate that there'll be more sales and the price will drop by a couple of per cent in the coming days/weeks, and delay their buying in order to buy the dip.

I'm sure the Americans have laid plans for how to avoid a major Oops.