Because I don't see the need, tortoise. If the person that ultimately buys this book wants some assurances or guarantees regarding its restoration status, I'm certain that he/she and I can work out those details to our mutual satisfaction. And the reality is, that person is the only one who needs to be reassured.
Well that's only true once it sells. At this point all potential buyers would need to be reassured. and a 3rd party grade would provide that. It's your book, do what you will, but it seems to me that a $100 investment on a book you value at 5k would only help your chances. But again, it's your book, not mine.