The runways of New York, London, Milan, and Paris have been swept clean of the residue of Fashion Month, which means it’s not a bad time to look at the downstream effect on retail, as well as where and how shoppers spent their time and money in September. The secondhand market offers a wealth of data points: In the two weeks after Matthieu Blazy’s debut Chanel collection in Paris, sales for the brand rose only 3 percent on The RealReal. But searches for “Matthieu Blazy” were up 83 percent. The discrepancy suggests that consumers, like many industry insiders, were taken with Blazy and his new direction for the brand, which Lauren raved about earlier this month.