Just curious, why do you think that? Is it because the book is overgraded, or is it that the book wil have a lower value simply because it is not CGC-graded?
Based on recent sales, looks like Tec 27 is roughly $120k per point now. In that case, a CGC 3.0 would be a $360k book. If this one sells @ CGC 2.5 price, then that would be closer to $300k. I would expect this book to pull a 3.0 from CGC, so the buyer would stand to gain $60k in "value" if the book's price is depressed because of the CBCS label.