there's used stuff sites like craigslist.com, and for gear there's musicgoround.com (might be music-go-round.com) but other then that there aren't many successful auctioning sites.
I'm sure there are a hundred auction sites, but nothing with as many sellers or buyers as ebay.
Ya , a few years ago there were a few, but Ebay has the e-auction market cornered.
The only other one I know of that is as big is a gun auction site, and that is only because Ebay dosent do guns.