Hey, Elizabeth Seward in a really good article has some really good advice:
If you have free things and want to get rid of them without contributing to a landfill, Craigslist is the place to do it. If you want help scoring that perfect telecommuting job, Craigslist is the place to do it. If you want to find a space to host your charity event, a renter for the extra room in your house, a clown for your kid's birthday party who will also take out the recycling—Craigslist is the place to do it. This website can open green doors you may have never thought existed.
Also, there's some good caveats regarding real-life transactions that start online.