Geo, instead of inscribing the book, maybe just write a personalized letter to your grandson, fold it over in the book. Then, maybe have some instruction that he gets that book.

I've had authors write personal notes in books to my boy that I hope some day he will find when I'm gone. But, if the folks just want to dump my stuff, it would likely be worth more without the writing inside. No big deal, sounds like it was a great day.