TORONTO, Sept 17 (Reuters) - Canada's main stock index slipped in early trade on Monday in a broad-based sell-off after several strong sessions of gains late last week on the back of the U.S. Federal Reserve's fresh round of fiscal stimulus. The Toronto Stock Exchange's S&P/TSX composite index was down 15.35 points, or 0.12 percent, at 12,484.87 shortly after opening in positive territory.