Yahoo earned itself a headache this week when more than a million people around the world signed a petition on the activist network Avaaz calling on the company to ban all sales of ivory on its Japanese auction website. It turns out that more than 12 tons of ivory exchanged hands from 2012 to 2014 on Yahoo Japan, according to a report released last December by the Environmental Investigation Agency.

With more than 30,000 elephants being killed for their ivory each year, Japan has emerged as one of the drivers of the illegal trade.

Here’s the catch: some of those Yahoo sales were probably legal. In Japan, as in most U.S. states, it’s permitted to buy and sell ivory imported before 1989, when a ban on international trade in ivory was passed. Yahoo’s U.S. sales website, just to be safe, has banned all ivory sales, legal or not. But Yahoo Japan continues to allow ivory sales. And because Yahoo in the U.S. doesn’t have a controlling interest in Yahoo Japan (a joint venture with Japanese firm Softbank Corp.), it says it can’t do anything.

Online ivory sales are worrisome because anyone can claim that the ivory they’re selling is legal—there’s no way to verify without official documentation. Laundering illegal ivory by calling it “antique” is just too easy.

Even with a ban on all ivory ads, combating ivory sales online is a challenge. There’s a constant whack-a-mole with code words, for example. As soon as a company (or law enforcement agency, for that matter) catches on that “white gold” is code for “ivory,” a new code word pops up. And there are just so many ads. How do you monitor them all?

We looked at the records of five major online marketplaces.

Google: Google came under fire in 2013 for not removing ads for ivory products on its shopping site in Japan. Most of the ads were for hanko, traditional Japanese stamps used as signatures on contracts. Ads for ivory are not allowed on Google, and the company says it deletes them as soon as it’s alerted.

Google took a step forward last year, when it adopted an algorithm an employee developed in her free time to figure out whether posts on Google Shopping and Google Ads were trying to sell real ivory. It looks not only for obvious keywords such as “ivory” but also for code words, word patterns, and other elements to identify ivory ads disguised as something else.

Yahoo: The company released a statement in response to the campaign to ban ivory on Yahoo Japan this week, reiterating its ban on ivory ads in the U.S. and distancing itself from Yahoo Japan. A spokeswoman did not respond to questions about how the company monitors and enforces its ivory ads policy.

eBay: A 2008 report singled out eBay as the worst offender in the online trade of endangered wildlife products. The International Fund for Animal Welfare, a wildlife conservation organization, found that 4,300 of 5,200 tracked elephant ivory listings took place on eBay.

The company has always banned the sale of endangered species products, but before the report’s release it explicitly banned the sale of ivory on its platform. “It’s a challenging effort and that’s why it’s something we don’t do alone,” says Mike Carson, eBay’s senior manager of global policy and regulatory management. Carson says that eBay enlists the help of NGOs to educate it about new trends.

Amazon: Like Google, Amazon explicitly bans ivory sales. Still, the Environmental Investigation Agency found in 2013 that the company allowed thousands of ads for real ivory on its Japanese site. The group presented its findings to CEO Jeff Bezos and eventually most of the ads were taken down. Now, EIA says, Amazon has a pretty good record of keeping ivory ads off its sites.

Craigslist: Craigslist was thrust into the spotlight in March 2015 after an investigation by IFAW and the Wildlife Conservation Society showed more than 600 elephant-related items worth a total of $1.5 million were advertised on the website. Many of the listings claimed the products were “pre-ban,” but only 21—3 percent—of all products came with any documentation.

Craigslist has since added ivory and endangered animal parts to its list of explicitly banned items. But a quick search yielded multiple advertisements for ivory products, including Chinese figurines in Maryland and an ivory necklace in California.