Yes you can buy and sell anytime you want because thats your preference, but i think it is still important to consider its price when buying or selling. Say for example you bought crypto at high price then sell it at lower price, then it will turn on losing all your invesments, that is why monitoring its price is very important and there is always a right time to buy and sell.
Never to sell when you are still at loss not unless the coin that you are holding already proven being scammed, but if you are holding good alts or you are holding bitcoin, its always far better to sell when you are already in the green position, the fluctuations really hurting our investment but it doesn't mean that we will going to allow losing our hard earned money.