N, R at 8th St.-NYU; 6 at Astor Pl. Hang out for a while at Flight Club's nondescript Greene Street location and it's easy to understand why this concept store is giving eBay a run for its money.

The process is simple: Flight Club lets the seller set the price, and the store takes a cut of the final sale. Flight Club also authenticates each pair—all without the shipping, payment ...