I think it's also the buyer's responsibility to ask questions and have a rough idea on how to grade comics, especially a comic of this magnitude. If you're going to be spending on an item like this, you better do your homework first. Like with most dealers, they cannot guarantee what a 3rd party grading company would grade the book, which I think is fair game.
I asked Sparkle City questions about the book and felt that based on their responses, that their assigned grade was a bit high. I actually had the book graded at a GD+ so was accurate there but didn't expect the light tan pages, which was way off from their offwhite-white pages they had it at.