I don't see it on eBay anymore, so I'm guessing you've pulled it down by now.
Demo Kits are typically Test Kits without sidecars. They're essentially development units that tend to be used in tradeshows because they look like a retail console, but are running a development kernel under the hood. They don't scare away consumers, or get people asking questions.
The $150 offer is a little low, but it really depends on the market. I feel it could fetch around 300 to the right person, so your 275 pricemark was a good target. Whoever you're having the back and forth with is just trying to get it out of the public eye, and nab it at a good price. You could sell it now at that price, or wait till someone with a bit more cash comes along.
Best of luck,
-NHN!