Just thought I'd give you some suggestions for next time, since I just sold some old PC's on Ebay recently. First, like others mentioned, custom desktops don't sell very well on Ebay unless it's very high end. And unfortunately, yours isn't. As a gaming PC these days, I'd call yours low end, especially with a 7900GS. Dual cores are cheap, 8800GT/ATI 4850 are ridiculously cheap, you can build one around that for around $350 and those video cards are 1 to 2 generations better than your 7900GS.
You offered Ubuntu instead of Windows. People who would consider buying a PC like this won't know how to use Ubuntu. And even if they figured out how to nuke it with the disc you're including, they won't know how to install and setup Windows. People who know how to use Ubuntu will probably build their own box.
People who don't know how to build would rather buy a named OEM builder like Dell and HP. Why? No idea, maybe they feel there's at least customer support when buying from those companies. Maybe because everything's set up from the get go. But when you buy a custom desktop off Ebay, who's going to help them with technical problems? No one. So that's why I believe mid to low end customs don't sell on Ebay.
Also, your feedback rating and those very dark pictures certainly don't help. I'm sure there's a good reason behind the neutral feedback, but unfortunately, people aren't going to shell out $449.99 to someone who's feedback on the page is 0%. And they're not going to take the time to click on your feedback and see that you only got 1 neutral, not a negative. Sadly, Ebay managed to destroy sellers' feedback with their new system in one day.
I had a custom PC for auction hoping for at least $150, just to recoup the costs of the parts. It ended at $85. And I had started it at $0.99 hoping that people would jack it up over a 5 day time span. I sold another old Dell XPS I had with roughly the same specs and that sold for over $300. Go figure.