Be a Message Board Regular
A few major sneaker sites have message boards dedicated strictly to a classified-like system for people to buy, sell, and trade. Use the boards wisely, build up a name for yourself, and help out where you can. You can hit up spots like Nike Talk and Sole Collector, but again be cautious and make sure homie on the other end isn't trying to rip you off. You could end up trading a bad decision from the spring for a new release you didn't have the funds for Saturday. Craigslist might be another lane, but beware.