Not good to buy when in bull run. Better to buy low and sell high if not you can lose big chunk of money.
As long as you gain profit and the price increase after you invested, you can sell without lose and hesitations. You have to consider the price of buying and selling even every time is good because some opportunity comes after you take actions which make you regret much.
Short term traders really loves to take advantage of the bull run, as they mastered the market sways and take each opportunities that been provided to them, buy while the bull is on its run and sell before it reached the peak before it dumped again, but before you can deal with this type of trading you needed to have enough skills because its really indeed to risky process.