Retail sales posted the biggest gain in six months in September, the government said Friday, helped in large part by strong auto sales as auto manufacturers enticed customers with more juicy incentives. The Commerce Department reported a 1.5 percent increase in retail sales in September, compared with a revised fall of 0.2 percent in August. It was the best showing in the core reading since a 0.9 percent gain in May. “This says the U.S. consumer is still in the ball game and playing like a champion,” said Drew Matus, senior economist with Lehman Brothers.