On the same day that Amazon started selling Harry Potter e-books in a landmark deal, buy buttons on the company's dominant Kindle e-book store disappeared for several hours.

Top-selling e-books, including The Hunger Games series and the Fifty Shades trilogy, instead had bigger green buttons saying "This title is not available for customers from: United States."

Several Amazon customers complained about the mysterious button disappearance on the company's online Kindle forum.

"What does this mean? Why am I seeing this message? I am from the United States. I want my readers who live in the United States to purchase this book. Any ideas why this is showing up?" someone wrote on the Kindle Direct Publishing online forum.

Amazon spokespeople did not respond to emails and phone messages seeking comment on Tuesday. However, a forum post from Amazon's Kindle Direct Publishing unit said it was a "website issue" affecting the buy box for Kindle books and noted that it should be resolved "shortly."

By Tuesday at 3 p.m. Pacific time, the buy buttons were back on listings for The Hunger Games series and the Fifty Shades trilogy titles.

(Reporting By Alistair Barr; Editing by Phil Berlowitz)

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