Is the stock a bargain, or just a value trap?Buying a stock on weakness can be a dangerous idea. Imagine a stock that you think may be a decent buy falls to a new 52-week low. Then a few weeks later, it hits a new low. Then another. And after a while of this tailspin, it's at a multiyear low. That's what many investors have been experiencing with Walgreens Boots Alliance (WBA -0.28%).Shares of the pharmacy retailer are not just near their 52-week lows but they recently even hit 27-year lows. This kind of an ...