Harry Potter and the Philosopher's Stone
Copyright - Joanne Rowling not J.K Rowling, a finer detail of the
Paperback 1997 first edition. Although your book is a first edition it is the 8th print run indicated by the 10, 9, 8 on the bottom of the publication page. I am afraid this does significantly reduce the price but the signature is a real bonus. It would be in your best interests to contact a book department at a well known auction house to authenticate the signature before sale. sadly many have been faked and she actually changed her signature a few times which really doesn't help!
Auction estimate £1000
Harry Potter and the Chamber of Secrets
First edition, 1998, first printing
Auction estimate £150
Harry Potter and the Prisoner of Azkaban
First edition 1999, second printing
Auction estimate £70
Harry Potter and the Goblet of Fire
First Edition 2000
Auction estimate £30
Harry Potter and the Order of the phoenix
First edition 2003
Auction estimate £10
Be aware book dealers prices are not an accurate comparison to a domestic seller.
If you are wishing to sell I would suggest an auction house such as Bonhams or Bloomsbury's
If you have any further questions please do let me know.