Herbalife (HLF) shares continued to tumble Monday, days after a noted short seller's remarks sparked a sell off last week. Shares initially rose in premarket trading after the weight-loss products maker said it will buy back more shares than the $50 million previously expected. Herbalife shares was down 6% to 25.75 intraday Monday. On Wednesday, Bill Ackman accused the company of being a "pyramid scheme" that has "grown remarkably rapidly" without