StockX, one of Detroit's fastest-growing startups, has become for sneakers what the Kelley Blue Book is for cars. : StockX, one of Detroit's fastest-growing startups, has become for sneakers what the Kelley Blue Book is for cars. The company is up to 370 employees and is handling $2 million a day in transactions. It launched just over two years ago. JC Reindl/Detroit Free Press Antonio Gray, 29, of Detroit, worked as a sneaker salesman for six years before becoming a authenticator at Stock X, where he looks over a pair of Jordans at their new operations center in Detroit.