Need a bike? Want to buy a nicer DVD player even though your old one works just fine? This year I have made an effort to buy used when I can, and sell my stuff when it is in good condition. When you're not in a donating mood, check out a few online and offline solutions.
The most obvious way to buy or sell stuff is through Craigslist or Ebay, etc. I am also a fan of Half.com for buying used. If you know of another good website, please list it in the comments. The downside to buying online is the wasteful packaging required to ship anything through the mail and the energy used to drive across town if you sell to someone nearby.
To avoid using extra packaging or energy, you can post a flyer in your apt building or a local coffeeshop or utilize (or start) a classified ads at your school, office, of place of worship. My office has an online Outlook forum to post about things you want or things you want to sell (you could start a blog on your intranet or even an LJ community to do this). I prefer to buy and sell things there because it doesn't require paper or packaging, and since all employees already drive to the office, it won't use up extra gas.
The extra bonus to this approach? You end up with a bit of extra cash in your pocket when you sell, and you get some GREAT deals when you buy. I got a $1300 bike used for $200 and a $500 couch used for $100 from my office classifieds.