go to this forum site: http://photography-on-the.net/forum/.
I'll second this. Have bought and sold a LOT of gear there, with great success. I just sold my 100-400 yesterday, for the same price I paid for it (listing was up for less than a day). While back, I sold a 50L (again, for the same price I'd paid about a month earlier), in 22 minutes.
When you're brand new to the forum marketplace, people can be a bit wary of you. Once you've built up some feedback, it's no longer an issue. Also, on the buying side, it's very important to check their feedback - I've never been ripped off, but every now and then of course, you hear stories