I was recently pointed to a site called GearTrade that looks like a promising alternative to the big (and sometime pricey) auction sites like eBay. Some of the unique advantages include no listing fees and long term listings (year-long). Right now there is little climbing gear, but the potential is there. As the site gets more exposure and traffic, GearTrade could turn into a winner.
From the site:
GearTrade had its beginnings when skitrade.com was first registered in September of 1998. Since then, the original idea expanded and evolved into its current form. GearTrade was co-founded in June 1999, in Provo, Utah, by a group of outdoor enthusiasts and entrepreneurs who noticed a need for a convenient marketplace for individuals and companies looking to buy and sell used and retail outdoor gear.
These outdoor enthusiasts realized that the problem was not a lack of gear, but simply that there was not an efficient marketplace for the distribution of closeout and used outdoor gear to the masses. GearTrade overcomes these inefficiencies and streamlines the entire process of bringing all the sellers and buyers of used and closeout outdoor gear together on one web site.