You know that Facebook ad offering free stuff to women over 40? Or that website promising free samples of your favorite perfume? What about that coupon you got via email for a shopping spree at your local grocery store?

Buyer beware: “Free” often comes at a price.

Legitimate companies sometimes offer loyal customers coupons, samples or trials in order to promote a new product. But there are also scam artists who go “phishing” for people's phone numbers, credit card numbers and other private data, using bogus offers as a lure. Once you take the bait, they may try to sell you financial products of dubious value or even sell your personal information to other scam artists.

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A popular grocery store chain recently posted on their Facebook page a warning against a fake Facebook page that offered free $100 store coupons. Instead of a coupon, consumers received offers for credit cards and subscriptions, according to the Atlanta Journal Constitution.

“The most captivating word, the one that gets the most responses in virtually any advertising, is the word 'free,'” says Milton Pressley, DBA, marketing professor emeritus and S.D. Leyda Fellow at University of New Orleans. “When they say 'free sample,' they’ve set the hook, so to speak, in much of the population with this kind of phishing.”

Even if you get hold of a coupon, it might not be authentic. In May, the U.S. Justice Department arrested an alleged counterfeiter, charging him with creating phony print-at-home coupons, which he sold on the infamous Silk Road black market website. One coupon supposedly allowed the holder to purchase a $50.00 VISA Gift Card for only a penny. 

One social media presence with a sketchy reputation was the Women Get It Free page on Facebook. Over the years, site visitors posted numerous complaints about suspicious charges on their bank accounts, unwanted calls from telemarketers, email spam, malware installed on their computers, trial offers that they had to call to cancel and few — if any — actual free products.

The WGIF page was created by the marketing firm Zeeto as a portal to its website getitfree.us, which has also been the subject of similar user complaints. In January, Zeeto spun off its social media unit into a new company, DapperToast, which removed the page from Facebook in July, possibly as part of its announced plans to merge WGIF with another website, JumbleJoy.

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Zeeto’s vice president of operations, Shayne Cardwell, says the company undertakes measures to protect consumer privacy. Examples include using fictitious customers to monitor the ethical behavior of their advertisers and sending a site visitor's personal information to a third party only if the visitor explicitly authorizes it.

“The user experience for us is really important,” says Cardwell. “We understand the user confusion and work really hard to assure that things are clear and conspicuous and explicit.”

How to protect yourself

Be savvy in order to be safe.

Don't take candy from strangers. Real offers, as opposed to phishing scams, typically come from companies with whom you've already done business, according to Pressley, who is also a private consultant and expert witness in marketing, consumerism and business ethics.

“Unless you recognize the company and remember checking a box where you agreed to receive something like 'special offers' in the future, don’t respond” to emails or other solicitations, he says.

Don't take brand name candy from third parties. If the offer comes from a third party, you're not being asked to do business with your favorite shoe company but with someone who's using that company's name to get your attention.

If you receive an offer via email, look at the email address to see if it’s from the actual company you know and trust, says Pressley. “If you see something like ac.at or tel2.com ac.at or tel2.com — something that might indicate that it comes from outside of the USA – I’d recommend not responding.”

Carefully check the coupon. If it doesn't have a manufacturer's bar code, the name of the store where it can be redeemed and an expiration date, it's likely to be phony, according to the Identity Theft Resource Center.

Visit the Federal Trade Commission to see an offer for a free-pizza coupon made to look like it comes from a well-known pizza company. Clicking on the coupon installed malware on your computer.

Related: 5 Ways to Avoid Computer Viruses

If they can't spell it, don't buy it. One clue that a fly-by-night overseas operation created a supposedly U.S.-based offer is bad spelling.

“It should look like it was written by an American who’s reasonably fluent in our language,” says Pressley. “If it’s written worse than the instructions on how to assemble something made in China…I won’t respond.”

Don't pay for free stuff. One scam involves offering a free item but requiring the customer to pay for shipping. “Under no conditions should you give out more than your name, address and phone number,” says Pressley. “Once you’ve given them your credit card number, expiration date and CID, you’ll probably see some charges you didn’t make on your next credit card bill, or the one after that, or both.”

David Arv Bragi is a freelance journalist and marketing consultant. He has been writing about health and safety issues since the 1990s and currently lives in Portland, Oregon.