Market downturns are rare opportunities to buy excellent stocks that rarely come cheap.Stocks generally don't move in a straight line. Yes, countless technology and artificial intelligence (AI) stocks have enjoyed a pretty straight path to higher prices since the beginning of last year. But recent volatility has reminded investors that even AI stocks are prone to a sell-off. Are you a long-term investor looking to own great stocks for years at a time? If so, a sell-off is a gift!There are businesses behind ...