The problem is you may be thinking an huge pump is over, then you sell, and it goes %100 from there. This happened to many people that sold at around $7500, which was the expect best possible scenario in 2017 by some traders with big followings like Tone Vays. Needless to say these people now hate themselves since they missed the peak to $20k.

Of course this is all very difficult to time. You just can't know where the limit of a bubble is, it is a gamble, so I don't see why you necessarily have to sell, just sit through it, and you will also save a ton fo tax-related headaches.

If with Bitcoin timing peaks is a bubble, imagine with altcoins, it is a total casino. So stick to Bitcoin and buy the dips. Even if it goes lower that where you buy at, you know you will not be holding a deprecating bag, contrary to what happens with 99% of altcoins.