Craigslist
I have had very good experiences with Craigslist (MD/DC/Nova). I've sold an alMBP 15", uMBP 15", iMac 27", Gateway 15" laptop, hackintoshed Dell Mini 9, and two gaming computers. I've bought my current uMBP 17" and 13" MBA, plus several of the aforementioned machines.
Some things to keep in mind:
* You will not get a "good" price for your laptop. I bought my 13" MBA (2010, 1.83GHz, 2GB RAM, 128GB SSD) for $800. I bought my 17" MBP (2009, 2.8GHz, 4GB RAM, 500GB HDD) for $925. Basically, expect to sell for 20% - 25% under refurbished prices.
* Expect to haggle and be lowballed. DON'T GET DISCOURAGED. When I sell, I typically settle about halfway between what someone initially offers and what I originally listed as the price. Experienced, serious buyers will attempt to haggle. Take this into account when pricing (but don't make it too high).
* Expect the sale to take some time. Apple products are popular, but even so, getting the deal you want can take time. The fastest I've sold anything is three days.
* For desktop transactions, it is generally accepted that the buyer will travel to the seller's home. For a laptop, this is more fluid, and it is not unreasonable to meet halfway. As the seller, make sure the laptop is fully charged. The buyer has the right to spend some time going over the computer and checking its specs.
* Take reasonable steps to protect yourself. This includes having a non-threatening buddy with you or nearby and purchasing a counterfeit bill detection pen at a store like Office Depot. Since many people pay with large bills for items >$100, it's never a bad idea to inspect the bills. The buyer should not be insulted by this (and none ever have, in my experience).
* You can expect the haggling to end once a place and time for the transaction have been determined. If someone tries to haggle or complain about something unreasonable once you have met in person, you have every right to walk on the deal.
* On that note, be as clear and specific as you can when listing an item. Pictures are a must. People are very particular about their Apple products, so make sure you mention tiny scratches if they exist. Don't say "flawless condition" and be surprised when someone bails on a deal because there is a small dent in the corner. Be as open as possible about issues, so the buyer has only themselves to blame if they did not read your ad carefully.
* People suck. You will deal with jerks, losers, uncommunicative buyers, people who show up late, and people who talk for forever about irrelevant and/or uninteresting crap.
* People are awesome. I've met some really nice guys buying and selling through Craigslist, and some whom have even become business contacts.
Good luck!