Thank you for sharing the story. Very nice to use the funds for a down payment for a house for your son.
Make sure to share a picture of the book here. I'm sure many of us fans would love to see it.
Personally, if I had a book like that to sell, I would look at one of the comic experienced online auction houses. While the auction house will also get a piece of the pie (very normal of course), I just would want to know how high it would go in a specialized auction. (It's also a little bit the gambler in me)
Whatever you do, good luck with the sale