The best way of studying the market on eBay is to look at Completed Lisings for Harry Potter first editions. Your feedback score suggests that you are new so forgive me if I'm doing things with my Grandma, eggs and slurping noises.
When you Search for Harry Potter, the Completed Listings is a box to tick on the left hand side of the screen
Good Luck
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"Write your Life Story and Publish it" .....
"Longdon Green, Farewell"....
"Use Google's Picasa for a Kindle, e-book, cover picture." ... on Hub Pages
Use quotation marks when searching.