You can basically just go through the old threads on M&A in regards to the camera question. Everyone will have a different answer, but you can cross-reference with Google (and find reviews for gear).
After that, if you do want to buy used, check out http://keh.com
. They have a great way for grading used gear. I've had many people tell me that they are very reliable, but have never bought from them myself. Best of luck!
"If you aren't crashing, you aren't skiing."
Sun Valley, Idaho