While the information architecture is fine, the site is really ugly. There has to be something functional and beautiful, and not just functional and bare like craigslist.
The rating system is idiotic -- an early form of karma whoring where the marginal cost of high ratings is too low. Perhaps it would make more sense to gain a point a per day or hour, and a 10 point rating system, where you need to allot scarce points when giving reviews.
The payment problem is solved with PayPal and others, so that is a non-issue. People actually trust it.
Better templates that allow for easy and beautiful sales pitches would be nice. Some of the 1996 era junk makes me want to run for the nearest far-more-pretty myspace page.
Community would make a lot of sense for particular items, like bobble-head dolls and replica light-sabers. Buyers of one rarely have the other, which beckons for sub-culture. Though I did know a guy who had mock light-sabers and a collection of StreetFighter 2 bobble-heads.