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I think it has become a natural thing that cryptocurrency price movements will always go up and down, you only need to take advantage of the moment of coin price movements when they fall then you can buy them and when prices go up you can sell them, you should not be affected by buying coins when prices are high because it can make you stuck at expensive prices and make you lose because you can not use your trading capital.
Is it possible to know which prices we should consider "high"? For example, in the beginning of September 2017 the price of BTC reached $4,600+, which was a great improvement in comparison with the previous ATH, $1,150. But was it the right time to sell? Hardly. Because the price was $19k+ just 3 months later. In short, I think that selling more than 50% of what you have is never a good idea.