Buy and sell totally depends on you but i always try to buy in deep and sell it high though we all do same mistakes in everytime but for selling your coins you can follow markets conditions, fundamental news and technical history too.
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I rather choose the keep buying than selling.Kowing the fact that when you sell is a very simple way, from the start of your investment in BTC, you should write it down in what price in the market you have purchased your BTC and then, just aim an amount that you want to sell your BTC, in that strategy, it is impossible you did not know when is the perfect time of selling. I also suggest, don't be too much greedy in making a profit in crypto, because sometimes it will cause your losses.
That only if you have a lot of money. But how about those poor people (third world country)? How about those you are just in short-term holding? This is not all about buying and holding, selling is a part of trading, and this why the market just moves up and down. And besides, we have the volatility feature because of this, some are selling and buying. Yet, both of them can be applied depending on our decision, and to ask when is the perfect time to do it(buy/sell), that gonna be a big question. It is just us to decide.