I have been using AutoCAD ADT 2005 at work, but we are now switching to Revit.  However, I would like to have a copy of ADT 2005 for my personal use at home, and was wondering if anyone could tell me where I might find a used copy (obviously at a reduced price!)?

The AutoCAD license agreement does not allow to sell a used copy of this software.

Why don't you just ask your boss if you can install ADT on your home computer? Since the licenses will not be used in the office anymore, you can just use the Portable License Utility to transfer a license to your home computer. That way, it won't cost you a cent.


 
See here: http://www.cvis.com/mp/using_the_portable_license_utility.htm