Buying or selling used outdoor equipment was once a major hassle. Other than yardsales, placing a classified ad was the only reliable way to trade used gear, and for smaller-ticker items the cost of the ad itself might easily eat up any profit. But—hurray—we now have the Internet. And everything is easier.
For years, I've been buying and selling bicycle gear on ebay. It's my go-to site for components and wheels, unless I can find on-sale parts at an online cycle shop like Excel Sports. But for general-purpose outdoor gear, like this Sierra Designs ASP 2 tent ($149 for buy-now, less than half the original retail price) ebay is probably your best bet.
Believe it or not, I actually use Craigslist quite a bit, too. That's mainly a function of how many people are on the site, and in my area there's a great selection. I once found an Osprey pack for $175. It'd been out of production for for years, but it's one of my favorites. Offer $125 and you'll get a hell of a good pack for a great deal. Another benefit to Craigslist, which is organized by state and town, is convenience. Once I find something it's never far away.
I should also mention the purpose-built sites for outdoor gear, like GearTrade.com. Prices seem comparable to Craigslist, but because it doesn't get much traffic, only a few buyers have high ratings, which makes me a little uneasy. You may also run into selection issues—I couldn't find any canister-type camp stoves, although there were quite a few packs and sleeping bags.
You might also take a look at Mountain Equipment Co-op's Online GearSwap page. MEC act as referee, lending the exchange credibility and providing a safe harbor for buyers and sellers both. You also can find people to hike with! A win all around.