Buying of coins is very easy. Anybody can buy and claim to be a trader. But the real issue is when to sell. When is the right time to sell after buying a coin? Sometimes you can be on profit and you will be tempted to leave your trade open for more profit and that's where the problem comes in. Once you are profit, sell off. Better with little gain than nothing a all. Don't be greedy when trading.
It isn't a problem. Greed can be cured. Obviously, you buy low and sell high, if it's done correctly there is no problem taking place. If (while already in profits) people leave a trade open with the hope to earn a few more percentages but finally didn't. It just a bad trade. Doing it repeatedly make you a bad trader
The easiest way greed can even be cured is when you have the knowledge and you know how to manage yourself well in the space, considering the fact that there is absolutely no way you can deal with your own emotions in the market when you do not even have a strategy in the first place.
When to sell, is always dependent on that strategy and how well you are able to execute them without any emotional attachment which greed could end up being one of those emotional attachments. What generally makes you a bad trader is trying to not let your plans work it out, but you just keep drifting over and over.