When you click Buy or Sell, the price you receive is locked in at that moment, regardless of market fluctuations between executing the buy or sell and receiving the digital currency. The price can be volatile - by locking in the price you always know exactly what you paid for your digital currency. You can find the price you bought or sold at on your history page.
Since the price of digital currencies are determined by supply and demand, there is a difference between the buy, sell, and spot prices displayed on the website. For more information on this please refer to this page.