On the flip side: I got a great deal on a gun once because "the barrel was bad". I brought it home and spent about an hour cleaning it out, and found that it was perfect. It was so damn dirty that it looked like it was worn out and pitted. So an unclean gun is not always a bad thing.
It's kinda funny though, you see dirty guns at the shows all the time. If you're trying to sell a car, you would more than likely wash it, get rid of all the hamburger wrappers and coke cans, vacuum it and so on. It just makes sense to make your merchandise pretty, it makes it easier to sell and you might very well get more money for it. Still, many people obviously don't want to spend a few minutes on cleaning a gun before taking it to the show.
