Back in 2004 and early 2005, I was hitting all of the internet classifieds, including truck related forum classifieds, on a regular basis. I forgot which one it was, but I found a good deal on a 2004.5 Dodge diesel dually with 5,400 miles on it from a private seller who was retired. I couldn't have been happier and glad I got it to replace my 1992 version!
I learned long ago to buy things a little better/heavier duty than what you think you need. Even the trailer I got is rated for 12,000 lbs despite figuring it would never be over 10,000 fully loaded. Gotta look at things from a safety standpoint as well as possible DOT issues just in case you ever get hassled, especially if your truck/trailer looks to be over the trucks tow rating.
It may have been this site:
http://www.kbb.com/kbb/Classifieds/default.aspx
Myron