EpicSki has a nice list of people buying, selling and trading used skis.

Gearswap by BackCountry Magazine is a list of ski items for sale and ski items wanted.

LiveToPlay does not sell used ski items, but instead sells new outdoor gear in an auction format.

ShopOutdoors.com offers some used ski and general outdoor recreation gear on this page.  Many items appear to be from California.

Skiers.com has a national list of skis and ski gear for sale.

SkiREVIEW.com's Marketplace and Classified section is a big list of for sale and wanted to buy ski items.

SkiTrade has a large national list of new, closeout, demo and used ski and ski gear items being listed from retail stores and individuals.

Sporting Auction sells used and new ski and outdoor recreational items in an auction setting.