A lot of Air owners (David included, and maybe myself) are selling them to buy new MacBooks or MacBook Pros. Like this.
They run new for $1800. Apple’s selling refurbs of the first-generation 1.6 GHz Air for $1350. I think a fair price for a used one is $1100-1300, depending on condition and any included accessories/AppleCare.
If you’ve always wanted a MacBook Air, this is the time to look for one.