Amazon, the online retailer with an increasingly broad range of products, has this summer been quietly pushing the limits on how far it is prepared to go.

Under the heading of Sex and Sensuality, discreetly located under the Health and Personal care section of its US site, Amazon is offering more than 40,000 products, including over 9,000 vibrators and more than 5,500 “sex-enhancers”.

The items are being sold through Amazon under its increasingly important third party retailer business, which accounts for more than a quarter of the items it sells, and which includes mainstream retailers such as Macy’s and Toys R Us.

Patty Smith, Amazon's spokeswoman, noted that some adult products have been sold by the retailer for more than two years. “It is just another of our selections for customers in all categories ... whatever you want to buy, you would be able to find it on Amazon,” she said.

But the range of more risque products on the pages, such as a “realistic” eight-inch dildo and the Optimum Power Thrusting Power Climaxer Masturbator, has dramatically increased this summer, with many of the items offered by Seakap Enterprises, one of the largest US distributors of sex toys. Seakap provides more than 15,000 items on the Amazon site.

Amazon says that it reviews the images that third party merchants place on its site, and that they cannot contain nudity, even on product packaging – although a survey of the site indicates that this policy does not extend to realistic representations of body parts. “Descriptions for the various products cannot contain profanity or graphic adult language,” Ms Smith added.

The pages carry some regular Amazon interactive features, such as lists of items recommended by other users, and the opportunity to review and rate products or to write users' guides.

But they do not include the “share your own customer images” feature that appears on Amazon's house and kitchen ware pages. And unlike other pages on the site, they do not send personalised recommendations to the homepage the next time a customer logs on.

Amazon has not drawn attention to the expansion of its sex business, which went largely unnoticed until Mark Morford, a columnist for the San Francisco Chronicle, wrote a column this month praising the retailer for “merely folding the toys into their massive array of general offerings”.

“What a desperately needed notion for a sex-starved and deeply misinformed, orgasmically uncertain nation,” Mr Morford wrote.