Based on no ETH/ZAR exchange I can only assume there is some calculation being made which is tied to the ETH/BTC exchange. With that said a coin value is normally stated as the mean price between last ask and first bid of the order book.
If you are doing instant sell, it will sell at the closest ask. If the ask is enough the order will complete but if not it will keep going down the order book until the order is complete. Long story short, don't go on the price you are shown as it's only an indication. This is why people are being burnt on the instant buy/sell.
Thanks, I'm not a Luno user yet so was wondering if the UI is nice enough to show what the ask would be that I will be selling at? I don't want to be burned.