Hey Purply,
No problem, happy to explain that one too :-)
If if the market is very liquid (lots of supply and demand), then yes, just 1% will be charged on the buy and 1% on the sell (or you can place your order as a limit order and 0% will be charged, but you'll have to wait for someone else to close your order --buy your Bitcoin or your rand you placed in that order).
But what you're getting at is still about the supply and demand (which isn't unlimited):
If you head over to the Luno Exchange (https://www.luno.com/trade/XBTZAR) you can toggle the "depth" view to see what the supply and demand look like in real time (or scroll through the order book on the left). This shows you all of the buyers (with ZAR) and sellers (with BTC) and how much they are interested in buying/selling.
If you look at the price of any financial instrument, say MTN shares, you also won't have an unlimited supply thereof. If you wanted to buy R10,000 of MTN stock, fine, there would be enough available at the market price. If you then immediately sold the R10,000 worth of MTN stocks for ZAR again, you would pretty much get the R10k back (minus some trading fees).
But, if someone came with a crazy amount like R500b ZAR, there simply won't be R500b worth of stock available at that price. So, the (crazy) buyer will eventually pay a higher and higher price on their stash of MTN shares until their money is used up. After their R500b is depleted, they could try to sell all those MTN shares in one dump, but again, there won't be enough of a market of buyers interested in buying it ALL at a single price and they'll lose a lot of money in the process.
Since Bitcoin is new (and really small, compared to, say, stock or forex markets) it means that with as little as R100,000 you could momentarily move the exchange rate. In my MTN-example above, what hedge funds do is to buy large amounts of shares over the counter (from a single seller at an agreed-upon price) or they stagger their purchases. I suggest that you always look at the depth chart (or the order book) on the Luno exchange before buying large amounts of Bitcoin in one go and to also stagger your purchases over a few minutes, hours or even days, depending on the volume.
More info on using our Exchange, here: https://www.luno.com/help/en/folders/11000004959
For the vast majority of customers, who just want to buy smaller amounts of Bitcoin, this obviously won't be a problem. Also, trading volumes and market depths just keep getting better as more and more people use the platform.