Amazon is not alone. In a survey of eBay, Amazon and Yahoo's auction sites, CNET News.com found dozens of auctions featuring weapons that are prohibited in many states, and goods the auction houses had supposedly banned from their sites.
Last year, CNET News.com found numerous auctions of illegal weapons on Amazon.com, which subsequently vowed to cleanse its site of such items.
Despite the stepped-up scrutiny, dozens of illegal items are for sale on the major sites. A search on Yahoo Auctions for "nunchaku" turns up 59 items. A search for "butterfly knife" yields nine items on Amazon's auction site and a set of six knives in Amazon's zShops. On eBay, one seller listed a martial arts weapons set, which included a pair of sais, a set of nunchaku sticks and three "dagger-type" throwing knives.
The auction houses have said that while they are too big to preapprove all auction items, they will remove objectionable items.
"We either catch this stuff proactively, or if someone brings it to our attention, we'll react immediately," Amazon spokeswoman Sharon Greenspan said.
Amazon says that it actively polices its site, while eBay says that it primarily relies on users to alert it to questionable auctions. Yahoo recently launched a "neighborhood watch" program that attaches a link to each auction, allowing members to instantly email their concerns to the company.
"Yahoo relies on its community of users to inform it of any activity that breaks its terms of service," a company representative said.
But some analysts and users say that isn't enough.
"It's too easy for someone to obtain one of these items," Gomez Advisors online auction analyst Martin Debono said.
The weapons auctions come amid increased scrutiny of the freewheeling nature of online auctions, with critics calling for sites to take more responsibility for what's offered for sale.
Last month, for instance, three leading video game manufacturers sued Yahoo over the sale of illegal copies of their games on Yahoo's auction site. And earlier this year, eBay bowed to outside pressure and banned the sale of items that promote hate groups such as the Ku Klux Klan.
Before selling items on any of the three sites, sellers must consent to a user agreement that either forbids the sale of specific weapons or prohibits the sale of illegal items.
"It's the responsibility of each user to familiarize themselves with our user agreement," eBay spokesman Kevin Pursglove said. eBay said it would suspend users who repeatedly violate the agreement.
But the rules can be confusing and are often buried within each auction site's help section. And they can be enforced capriciously. Amazon's Greenspan, for instance, said Amazon will allow items that may be illegal in some states, as long as the items are not illegal in most states.
Several sellers said they didn't know it was against the rules to sell their weapons. "I didn't see anything that said what you could and couldn't sell," said one seller who requested to remain anonymous.
Some analysts say the sites should go further, such as adding filtering software that might weed out such auctions or at least alert site administrators to them before they are posted.
"If you say that you're not going to allow something on your site, you are responsible for taking care of that," e-commerce analyst Jonathan Gaw of International Data said.
By not doing a better job to limit such auctions, analysts say the auction houses are playing a dangerous game. If an illegal weapon purchased online were to be used to seriously injure or kill someone, it could lead to a public relations nightmare, not to mention potentially disastrous lawsuits.
"They just can't let that happen," Rich Gray, an attorney with Outside General Counsel of Silicon Valley. "They're just asking for trouble if they don't police the stuff."