The S&P 500(SNPINDEX: ^GSPC) stock market index closed at 6,506 on Friday, March 20, marking a 7% decline from its all-time high. Sell-offs of this magnitude are relatively common, but with economic uncertainty on the rise and geopolitical tensions raging in the Middle East, there is a risk of further downside from here. But throughout history, investors who treated periods of weakness as buying o...