What's a good place online to sell a used Mac? Where can I buy or sell a used iMac online?

EDIT:

I have a 21.5" iMac (Core 2 Duo, late 2009) and a MacBook which is about two months old. I'd like to sell both and upgrade.
If you're in the USA, gazelle.com can be a good place, but the prices aren't always great.