StockX, the high-flying resale marketplace that connects buyers and sellers of sneakers, streetwear, handbags and other collectible items, has seen its fortune rise along with the $6 billion global sneaker resale market, which is part of the broader $100 billion sneaker category. In fact, the company, which was assigned a billion-dollar-plus valuation last year, says $1 billion worth of merchandise was sold through its platform last year. The big question is whether StockX can maintain its momentum.