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When Mary May started buying from third-party sellers on Walmart

’s online marketplace, she said she assumed the products she was purchasing were the same as the ones she’d long bought in stores.

So in late March when she said she saw a “ridiculous sale” on her favorite Neuriva brain supplements on Walmart’s marketplace, she bought eight bottles for her and her sister.

But when some of the once-daily oral supplements arrived from a seller calling itself Lifeworks-ACS, the 59-year-old mother of three noticed there were misspellings on the bottle and the packaging looked different than it usually did. Weeks later, CNBC confirmed the supplements were counterfeit – and the seller had taken the identity of another business to sign up for the marketplace.