You can set up a limit order to buy at a certain price. The problem is that it may never go as low as that price, so the order may never get filled. Or you may choose a conservative price and see it go 10 or 100 times lower and then wish that you had set a lower limit (like the latest crash). Your money will be tied up on the exchange the entire time that the order is in place, since you have to fund the order ahead of time, but you can cancel the order whenever you want.