In recent weeks, it was easy to observe the dangers of short-term investing. In the blink of an eye, the three major benchmarks -- which had climbed over the past two years -- suddenly found themselves in the doldrums. The S&P 500, the Dow Jones Industrial Average, and the Nasdaq Composite each slid in March and April on concern that President Trump's import tariffs would hurt economic growth.Investors who aimed to sell stocks they recently bought may have found themselves in a difficult situation. But the ...