SAN FRANCISCO — Amazon is quietly canceling its Nazis.

Over the past 18 months, the retailer has removed two books by David Duke, a former leader of the Ku Klux Klan, as well as several titles by George Lincoln Rockwell, the founder of the American Nazi Party. Amazon has also prohibited volumes like “The Ruling Elite: The Zionist Seizure of World Power” and “A History of Central Banking and the Enslavement of Mankind.”

While few may lament the disappearance of these hate-filled books, the increasing number of banished titles has set off concern among some of the third-party booksellers who stock Amazon’s vast virtual shelves. Amazon, they said, seems to operate under vague or nonexistent rules.

“Amazon reserves the right to determine whether content provides an acceptable experience,” said one recent removal notice that the company sent to a bookseller.

Facebook, Twitter and YouTube have been roiled in recent years by controversies that pit freedom of speech against offensive content. Amazon has largely escaped this debate. But with millions of third-party merchants supplying much of what Amazon sells to tens of millions of customers, that ability to maintain a low profile may be reaching its end.