I had two MacBooks that I recently sold. I did this:
1. Price the MacBook at about $100-200 lower than the local going Craigslist rate (which is usually a ripoff for some reason, especially here in Toronto)
2. Take good pictures.
3. Write clear, detailed "ad copy", for example, I put in mine how many months of Applecare left (and how it can still be extended), I put in that my MacBooks had NO DEAL PIXELS. I still had the original boxes, I put in that the computers were "purchased brand new, they are NOT refurbs". Keep in mind I told the whole truth in my ad copy, so if it ain't true, don't put it on there!
I put them up on Craigslist and both ads got responses within literally 15 minutes and they both sold within 1 day of putting up the ads. As far as what I got for price, I ended up selling them for about how much it would have cost for similarly spec'ed refurbs from the Apple Online Store. Considering that I usually know how to get a decent deal on a new computer, I look at it as I rented the computers for ~6-12 months for a few hundred dollars, imo that's a good deal and how I prefer to go about computers. I'd rather "rent" them like this and just look at it as a cheap expense when averaged out over time.