Have you shopped around locally yet? I know of at least one independent Pioneer/Premier dealer in your area that offers excellent service and very competitive pricing. They probably won't be as cheap as eBay, but you'll get the benefit of a usable 2-year warranty. Plus you might even get some free installation tips for your particular vehicle.

Personally I think that Audiobahn sub is a waste of money, but then again you're not going to find much else at that price point. If you do buy it, just be prepared to be wanting an upgrade or your trunk space back pretty soon. I would budget a bare minimum of $350 for an entry-level sub setup, including enclosure, amp, and a proper wiring kit.

Good luck.