The Hempcoin is available for purchase on numerous world-class exchanges, but what is an exchange and how exactly does it work?
Cryptocurrency exchanges are "businesses that allow customers to trade digital currencies for other assets, such as conventional fiat money, or different digital currencies" (-Wikipedia)

As Bitcoin was the original cryptocurrency almost all exchanges use it as the default trading currency. Some accept fiat (US Dollars, British Pound, etc.) but almost always it is Bitcoin. The Bitcoins are then traded to your desired cryptocurrency and vice versa much in the same way the US Dollar is used almost everywhere as the base for fiat currency exchange.