I was thinking the same about the EXIF as Randy - where the Serial Number of your camera is embedded in the image. Then I remembered that as I use RAW mode in my 20d, and I convert these RAWs to TIFs for editing in Photoshop...all the EXIF info is lost as TIFs dont store EXIF! You wouldnt therefore be able use a serial number in an image produced this way, and would likely have to supply a secondary conversion in JPG of the original RAW...which is bound to cause more confusion for the average sales person.