The P/E ratio is at a multiyear high.The stock of Costco Wholesale (COST 0.69%) continues to appear unstoppable. It has risen by more than 35% this year. Given the strength of its business and a recent membership price increase, the optimism surrounding this stock appears elevated.However, this situation leaves new investors wondering what to do. Are the continued increases a sign that the retail stock's valuation doesn't matter, or are investors who buy now setting themselves up for months or years of pain ...