Astromart
Everything for the astronomer, including telescopes, mounts, eyepieces, filters, telescope-making tools and other equipment.
http://www.astromart.com/

Sell
Contains used telescopes and accessories.
http://www.astrobuysell.com/

Chicagoland Astro-Ads
Listing of telescopes and accessories in the Chicago area.
http://www.stargazing.net/naa/ads.htm

Internet Telescope Exchange
Listings of telescopes for sale or trade.
http://www.itetelescopes.com