Retailer Amazon has removed several abuse-themed e-books from its Kindle Store after a report highlighted titles depicting rape, incest and bestiality.
Titles such as Taking My Drunk Daughter had been on sale.
Amazon took down the books found by technology news site The Kernel, but many others still remain.
Other online stores, including WHSmith, Kobo and Barnes & Noble featured books with similar themes. They are yet to respond to requests for comment.
The BBC found that on Amazon’s store, the search function automatically suggested explicit topics to users typing seemingly innocuous keywords – without age verification taking place.
Amazon has not responded to the BBC’s request for comment on the issue, except to confirm that the specific books listed by The Kernel had been removed.
The titles can be found in the self-published section of the retailers’ sites – an area where authors can offer their own work. The companies take a percentage of the sales made through their stores.
One lawyer told the BBC that the retailers could find themselves guilty of a criminal offence for allowing such content to be found without protection mechanisms.
“The directors of Amazon have a very difficult question to answer: why are they making profits from pornography which, on the face of it, seems to be criminal?” said Mr Stephens, former chairman of the Internet Watch Foundation, a body responsible for monitoring criminal content online.
However, many of the authors have taken measures to stay within the law, adding disclaimers to their descriptions, such as saying characters were “over 18” or “step-daughters”.
On Amazon, guidelines for self-publishing state: “We don’t accept pornography or offensive depictions of graphic sexual acts.”
It adds: “What we deem offensive is probably about what you would expect.”
The other retailers give similar guidance.
In July, prime minister David Cameron said the government intended to make it illegal in England and Wales to possess online pornography depicting rape.
But it is unclear whether the written word – currently governed by the Obscene Publications Act (OPA) – will come under the proposed legislation.
Under the OPA, publishers have a duty to protect the public from accidentally encountering material that could outrage public decency, said Mr Stephens.
A spokesman for the Association of Chief Police Officers told the BBC: “Rape is a serious criminal offence which has a physical, emotional and psychological impact on victims. It’s very unpleasant and distasteful to use such a harrowing experience as the basis for entertainment and enjoyment.
“Investigating offences of rape is a particularly complex process because it often rests on the issue of consent.”
John Carr, secretary to the Children’s Charities’ Coalition on Internet Safety, said parents would be “shocked” at what content was discoverable.
“At the very least there should be a certain class of material that is adult, which ought not to be universally accessible,” he told the BBC.
However, others felt that Amazon’s removal of some titles amounted to censorship.
“We outlaw snuff films, child porn and, increasingly, revenge porn, because actual people are harmed during their production,” wrote PJ Vogt on OnTheMedia.org.
“Erotic fiction concerns fake characters who don’t exist in real life.”
Mr Carr stressed that he did not condone censorship, but that the content needed to be walled off.
“If this was a Soho sex shop, I wouldn’t take the same view. I am concerned that this is next to things kids could search for.”