Imagine a situation where no one wants to sell their Bitcoin. What happens? Because the supply is low and demand is high, the price of bitcoin rises. Soon enough, the price will be high enough that many people who bought bitcoins when it was cheaper will decide to sell and make money off of it. Sooner or later, it could happen that too many people are now trying to sell off their bitcoins seeing this increase in price, thereby bringing the price of bitcoin back down again. Thats when you buy it.