The online auction house eBay Inc. said Tuesday it will ban cross-border trade of ivory products, following a study by a wildlife group that found nine out of 10 of the items sold on the Internet are probably illegal.

The company also will alert traders on its Web site that they may need to prove they are legally allowed to sell their ivory products, spokeswoman Nicola Sharpe said, speaking from San Jose, Calif.

The first report of the ban came at a conference in The Hague where regulation of the ivory trade is high on the agenda of the Convention on International Trade in Endangered Species, or CITES.

The International Fund for Animal Welfare, the British-based wildlife group that negotiated the ban with eBay (nasdaq: EBAY - news - people ), said the announcement should send a signal to the 171-nation CITES to tighten the trade regime to further protect elephants from poaching.

The ban would apply only to international trade, and not to sales within a country's borders, Sharpe said. Laws on domestic ivory sales differ from country to country.

"The Internet is a huge challenge," said Claudia McMurray, the U.S. assistant secretary of state for environment issues, acknowledging that the United States has a big market for illegal ivory.

While applauding the decision, McMurray said: "You still have to identify the buyer and the seller, and ascertain whether it's elephant ivory and legal." The United States was prepared to prosecute illegal traders, but "in cyberspace you don't know who you are dealing with," she told The Associated Press. The U.S. government has worked closely with eBay in the past, and it has been "very good at understanding how they can ferret out illegal behavior."

In a statement, eBay said the ban was intended to give "confidence to those people who want to buy legitimate and legal ivory items."

Peter Prueschel, of the animal welfare group, said it was difficult to distinguish between domestic and international trades on eBay. "That's why we tell them that sooner or later they will have to ban ivory entirely," he said.

Prueschel said eBay in Germany, which displayed an average 400 ivory items a day on its site, banned all ivory sales in March 2006. Since then, traffic has plunged 98 percent.

Only ivory that predates the 1975 CITES treaty or ivory from national stocks which later received one-time exemptions can legally be traded across national borders.

Some items mistaken or mislabeled as ivory actually come from walrus or mammoth tusks - increasingly available as the Russian and Alaskan tundra melt due to global warming.

In its study, the animal welfare group said it found 2,275 ivory pieces offered for sale on the Internet in seven countries during a randomly chosen week in February, and that 94 percent lacked any mention of certification and probably were illegal.

The report coincided with another study by Care for the Wild International that identified the United States as one of the world's leading markets for illegal ivory.

That report's author, Esmond Martin, said he found more than 23,000 items of ivory in a survey of 15 U.S. cities. In some cities, half the items were illegally imported, and bore the hallmarks of production in China.

He said increasingly the ivory trade was migrating from retail stores to the Internet, which is not regulated by federal or state law.