Web sites promoting products supposedly containing the antioxidant resveratrol and antioxidant-rich acai berry juice have been springing up across the Internet. But our advice on these sites: Buyer beware.

In January, the Better Business Bureau issued its own warning about Web sites using "misleading sales and marketing practices" to promote acai berry products. Consumer complaints to the BBB alleged unauthorized charges to credit cards and bank accounts and difficulty contacting the companies and cancelling subscriptions.

We’ve also noticed numerous posts on Web-based message boards, including the one on Oprah’s Web site, alleging similar problems in connection with the marketing and sales of these types of products, some in recent days. One of our staffers reported receiving annoying pop-up ads on her computer from companies hawking resveratrol products.

In a twist, some sites with official-sounding names display warnings about ineffective resveratrol products and then go on to list products they’ve supposedly approved. We’ve seen this trick used before with other marketing efforts, including on sites advertising opportunities for mystery shoppers.

To read more on these promotions and for our health advice on resveratol, see our Money blog.