It is very difficult to sell at high and buy at low. And this is difficult to do even for experienced traders, and it is impossible for beginners to do it at all. In this case, you need to sell your bags in parts and you will end up with a good average selling price.
The problem is nobody knows how long the market will remain bullish. Even experienced people get panicked when the crypto market goes down more than 10% in a day. An increase of 10% a day rarely happens, while dumping of 10% a day happens very often. Usually, the crypto market accumulates slowly, and when it dumps, it dumps suddenly. That is where most new people get confused and sell their holdings even if they are at a loss.
Everyone should have their target, and they should make a profit once they reach the target. For example, if someone had a plan to sell BTC at 100K, they should do it because the bull run won't be there forever. You will have a chance to buy back again.