Used computers have mediocre value at best. Especially if they aren't top of the line "Previous Gen" ones.
With new laptops being so cheap - selling a used one is only going to get you so much. Maybe you'll get lucky with a person not familiar with computers, but more likely you'll just get a bunch of low-ball offers. Everyone checks the ads for Best Buy / Circuit City in the Sunday paper and given the low prices there for new comps, you can see why the "used" computer business has ended up on the low end of pricing.