Maybe read my post again - I made a few Edits to it
I think it explains your problem?
Again, with "Quick Sell", you can only sell as many BTC at a specific price, as someone else is willing to pay for it.
If you want to sell 1 BTC at R12 000, but there is someone only willing to buy 0.5 BTC at R12 000, you will only sell 0.5 BTC.
If there is someone willing to buy 0.5 BTC at R11 950, then you ca sell your remaining 0.5 BTC for R 11 950 to that buyer.
There is not someone at the back of the "Quick Sell" option that will just buy your BTC at whatever price you want to sell your BTC at

. Its all supply and demand.
Have a look at the Exchange and you will see this with the Buy and Sell orders sitting there. The "Quick Sell" is just a GUI over the Exchange at the back.
Make more sense now?