It suggests that foreign powers could be buying military equipment to augment their own forces.
"GAO found numerous defence-related items for sale to the highest bidder on eBay and Craigslist," the report claimed.
"During the period of investigation, GAO undercover investigators purchased a dozen sensitive items on eBay and Craigslist to demonstrate how easy it was to obtain them. Many of these items were stolen from the US military."
Two merchants were identified who claimed that they sold items on the sites that were sold to them by serving members of the military, who had stolen them to sell on. Soldiers were also selling stolen items directly on the sites, it was claimed.
Most of the equipment available online is relatively innocuous, the report admitted, but some items, like radar antennas for F-14 aircraft and attack helicopters, are of more concern.
Iran currently maintains a fleet of obsolescent F-14s that were sold to the country when the Shah was in power. But Iran is unable to obtain parts directly because of an arms embargo by the US.
Craigslist and eBay came in for criticism from the GAO for not checking the provenance of items for sale on their sites.
EBay said that it checks some items but is limited in what it can do, while Craigslist, which has few staff, relies on users.
"When we asked a Craigslist manager about whether his company had a fraud investigations team he said: 'I am the fraud investigations team for Craigslist,'" the report stated.
"This official added that Craigslist relies primarily on its user community to identify suspicious advertisements and report prohibited item sales."