Japan opposed a proposed ban on the export of Atlantic bluefin tuna
Conservationists say the internet has emerged as one of the biggest threats to endangered species.
Campaigners say it is easier than ever before to buy and sell anything from live baby lions to polar bear pelts on online auction sites and chatrooms.
The findings were presented at the 175-nation Convention on International Trade in Endangered Species (Cites), which is meeting in Doha, Qatar.
Several proposals to give endangered species more protection were defeated.
Delegates will vote on changes to the trade in ivory later this week.
"The internet is becoming the dominant factor overall in the global trade in protected species," said Paul Todd of the International Fund for Animal Welfare.
He said thousands of endangered species are regularly traded on the internet, as buyers and sellers take advantage of the anonymity - and vast global market - the world wide web can offer.
Those trying to police illegal sales say the size of problem is almost impossible to estimate. They say the US is the biggest market, but that Europe, China, Russia and Australia also play a large part.
On Sunday, delegates voted to ban all international trade in a rare type of Iranian salamander, the Kaiser's Spotted Newt, which the World Wildlife Fund says has been devastated by the internet trade.
However, more high-profile attempts to ban trade in polar bears, bluefin tuna and rare corals have all failed, leaving environmental activists dismayed, the BBC's Stephanie Hancock reports from Doha.
A proposal from the US and Sweden to regulate the trade in red and pink coral - which is crafted into expensive jewellery and sold extensively on the web - was defeated.
Delegates voted the idea down mostly over concerns the increased regulations might impact poor fishing communities.