Costco (NASDAQ: COST) and Sprouts Farmers Market (NASDAQ: SFM) both look vulnerable for very different reasons. Costco is firing on all cylinders yet trades at a valuation that leaves no room for slippage. Sprouts is hitting the wall on comps while still expanding aggressively. The question for short sellers is which crack widens first. One ... Costco or Sprouts: Which Is the Better Short Bet Right Now?