Sam's Club, the warehouse division of Wal-Mart Stores Inc, has tied up with online car-shopping service TrueCar Inc to make a full-fledged foray into auto sales, a fast-growing business for rival Costco Wholesale Corp. Until now Sam's Club had allowed a third-party auto-buying service to market cars through its website but had not operated its own program like Costco, which has quietly grown into one of the biggest players in the industry, selling nearly 400,000 vehicles last year. Sam's Club said on Thursday that its new program would give members access to more than 10,000 TrueCar dealers and guaranteed savings off the manufacturer's suggested retail price for new cars as well as deals on used cars.