I'm in the same boat as you. I have my 32GB/AT&T iPad on Craigslist listed for $600 and all I'm getting are ridiculous offers. As much as I want the fourth-generation iPad, I refuse to let this upgrade hit me hard on the wallet. I'm willing to come down a little but I'm not gonna give it away either.
This one guy offered me $500 and I met him halfway at $550 and he insisted on $500. Screw that!