When the moment you want to buy or selling bitcoin, my opinion is that you just have to do analyze first the chart of the coin that you want to buy dont just do guessing, then you can check the price of all currency at the coinmarketcap. After that you can go ahead and buy some crypto you want and hold it if you think the price of that currency you choose has a potential to moon. Always monitor the chart wait until it reach the peak then sell if you think the the price of your coin is profitable.