As they enter the holiday shopping season, online marketplaces Inc. and eBay Inc. are fending off reports about products with high mercury levels being sold on their websites in more evidence that the convenience of online shopping can come with product safety risks.

Activist groups including the Sierra Club purchased 158 skin-lightening products from Amazon and eBay marketplaces in 12 countries — including the U.S. — and found that 60 percent were contaminated by mercury.

"One major problem is that none of the products list mercury as an ingredient on the label, so it is impossible for an individual to know the product could be deadly," said Sonya Lunder, spokeswoman for the Sierra Club, which partnered with Zero Mercury Working Group and The Beautywell Project on the product tests.