As an executive at Titan, a large out-of-home advertising company, Michael Palatnek had plenty of inventory—in the form of street kiosks, sides of buses, and subway-station walls—at his disposal. Still, clients sometimes wanted their messages plastered somewhere or on something Titan didn't control. Palatnek would point them toward whoever did. Then he started asking: If a single source could identify all the best outdoor spaces for your campaign and secure them—regardless of their owners—would that interest you? Well, yeah, said the clients. Encouraged, Palatnek launched C2C with David Stutz, his best friend since childhood. The two immediately began work on a software system that identifies thousands of advertising spaces, from almost 200 vendors, by location, price, and dimensions. C2C's growth tracks that in outdoor advertising, which has benefited from the proliferation of smartphones. "You're riding on Metro-North and see an ad for an interesting book. You pull out your iPad and order it right there," says Palatnek.