No foot in mouth there fhw.
I certainly agree with you that you do not need a double blind test to actually test but as you say, the person 'assisting' has to be completely mute during the process.
Often salesmen CANNOT remain quiet and you would still have to remove the visual cues from the testing perhaps with a blindfold. This might look a bit awkward walking into the a/v store though.

"Those who preach the myths of audio are ignorant of truth."