I think it some circumstances it could be. If there was a stock 60's Precision with some nicks & scratches (and perhaps a bit of buckle rash) I would consider that excellent condition for a 40 year old bass. Not "mint" condition obviously, but excellent considering its age.
But discussing a much more modern bass, I'd say it's because many people (especially in this day and age of being inundated with advertising) have accepted deception and half-truth as the proper steps to take when selling anything. Of course, as mike_v_s pointed out, it's a different ballgame when selling to an informed audience. Personally, when I sell bass gear, I find it best to be completely upfront about it.