[gruez]

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.

[xapata]

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.