I've had success selling on the AGF classifieds, Reverb, and Ebay.

AGF is cheapest but tends to take longer. Things sell fastest on EBay, but it costs the most. Reverb is right in between in both regards. I've moved a couple of bicycles on Craigslist, but have never been able to sell a guitar there.

To be perfectly honest, I don't like the face-to-face interaction or the inevitable haggling that Craigslist entails. I think it's best for smaller-ticket items.