If you go to the "shopping" tab on this site (listed above). You can compare all the prices between merchants and also read reviews on the merchants to find out how dependable they are. http://picturecorrect.pgpartner.com/home_photo.php/
Hey Dan,
Best used business on the internet would probably be KEH.com. I've probably bought $10k worth of used hardware from them over the years. Their business practices are first rate and their rating of used equipment is very conservative. I am in agreement with Keith about B&H. Very fast and professional. No love from them but solid otherwise. Don't forget Calumet. Develope a relationship with a sales person over the phone and you will get "hometown service" from them.
Mark
I used to also buy from Adorama, but got in a dispute with them when an item I paid for expensive next day shipping took 19 days to reach me. I politely asked that they credit me the difference between overnight and regular shipping; they hung up the phone.
For used I like KEH and Midwest Photo Exchange www.mpex.com