The listings for ammunition clips and boxes of bullets were uncovered by the Washington Post and a Twitter user. The products are legal for online sale in the US but violate the two companies' own seller guidelines.
Google this week took down a listing for a 100-round magazine spotted by Twitter user Pinboard, the Post reported. The Post also found listings for boxes of bullets on Google's Shopping site.
"Our hearts go out to the victims of gun violence. The sale of weapons, guns, and certain gun parts is strictly prohibited on Google Shopping," a Google spokeswoman said. "As soon as we found policy-violating results, we removed them and are working to prevent these instances from reoccurring."
Amazon took down one listing for an ammunition clip that the Post found.
"All sellers are required to follow our selling guidelines and those who do not will be subject to action, including potential removal of their account," Amazon spokeswoman Cecelia Fan said in a statement. "The product in question has been removed."
The presence of these listings points to how vast both companies' listings have become, with Amazon, for example, now posting hundreds of millions of items. That huge inventory increases the chances banned items can become available, even with these companies' detection software and human monitors.
This isn't the first time these companies have been called out for problematic listings., for example, was criticized for recommending customers buy combinations of elements required for making crude bombs.
Neither company has a blanket ban on all gun parts and gun accessories. While they don't allow the sale of guns on their shopping sites, they do allow the sale of accessories and parts that include gun locks, scopes and holsters.