If you just want to do a few onetime trades, try u-exchange.com or the bartering section of the classified-ad site craigslist.org. Swaptree.com specializes in books, CDs, DVDs, and video games.
One catch: As with any transaction involving strangers, you can get taken if you’re not careful. If you’re exchanging low-value goods like books and DVDs, you’re probably okay shipping them to the other person. But if you’re swapping valuable goods, you should trade only with local people so you can meet them in person and inspect the products you’re getting in return (you can’t always trust photos). Bring a friend with you and arrange to make the swap at a public place, like a coffee shop or a restaurant.