Guitar Classifieds (Buy & Sell Gear)

Ebay.com
Free for buyers. An easy way to find used guitar equipment. Always a huge selection from sellers around the world.

The Gear Page
A high traffic message board with an emporium section to buy & sell musical equipment.

Craigslist
One of the world's most popular free local classifieds website. There is a local page for most all major cities/regions.

Harmony Central
A long time resource for free guitar classifieds.