It sounds like such fun—mystery shopping! A marketing research company hires you to go into a store, make a purchase, and report on the experience. Whatever you buy is on their dime and you get to keep it. For your trouble, you’re given a small reimbursement.

Mystery shopping jobs can be legitimate—and there’s even an association of reputable companies trying to maintain the integrity of the business.

But a scam that got a foothold a couple of years ago is now going strong all over the country. People are reporting being solicited through the mail with authentic-looking checks for large sums of money as enticement for carrying out a “mystery shopper” task that usually involves wiring money to a foreign location. If you can’t guess what comes next, we’ll just tell you: the check is counterfeit, and the money that just left the country was yours.

There are many reasons the scam appears to be believable. The official-looking letter you receive often has telephone contact numbers and company names that may mirror the name of a real company. The check inside—which can be for as much as $5,000—often looks official as well and may bear the name of real banks or credit unions. Sometimes the letter says that as part of the paid training assignment, you’ll spend around $100 of the cashier’s check at familiar major retail and restaurant chains—such as Kmart, Sears, Lowe’s, Best Buy, J.C. Penney, or Burger King—in order to evaluate the services received. Though this adds an element of legitimacy, these companies are not affiliated with the scam.

The real problem arises when you’re told that your job will be to go to a specific money-wiring service (Western Union or MoneyGram, for example) or check-cashing outlet (such as Wal-Mart) and evaluate the business. Before you do this, you are to deposit the check they sent into your bank account. Then you are to immediately wire a large portion of that money—several thousand dollars—to a given location. But unbeknownst to you, the check you deposited was counterfeit, so it’s not going to clear. The scams works, according to the Michigan attorney general’s Web site “by taking advantage of the time-delay between when the cashier’s check is deposited and when the bank discovers that the check is counterfeit.” That could be several days or even weeks. But by the time the check bounces, the money you wired is already in the hands of the thief, and you’re stuck owing the bank a huge amount of money.

Some of the company names that have popped up in complaints found on the Internet include Master Research Inc., Frontline Consumer Research Group, Consumer Research Institute, Temp 1 Employment, and Secret Shoppers Inc.—although a company called Secret Shopper appears to be a victim as well and has posted a scam alert at its site.

Other legitimate businesses have been unwittingly getting caught in the scam. Pioneer Federal Credit Union in Idaho was drawn in when it learned that counterfeit checks were being issued with its name on them. It posted an announcement at its Web site that told how these checks differ from anything Pioneer would issue. The Consumer Electronics Association, which produces a huge consumer technology trade show called International CES, discovered that a company called International Consumer Evaluation Services LLC (also calling itself CES) was sending consumers invitations to be secret shoppers and using the International CES name and logo in its messages. The electronics association posted a message disavowing any affiliation with this other group.

Attorneys general across the country have been posting warnings on their Web sites about these scams. In response to their concerns, MoneyGram International in July announced steps to minimize the abuse of its services by fraudulent telemarketers. In cooperation with 44 states and the District of Columbia, it’s embarking on a five-year plan to raise public awareness of the dangers of fraud through consumer education, more prominent fraud warnings for money transfer customers, and enhanced training for agents. It will also contribute $1.1 million to the AARP Foundation for Consumer Education to fund a fraud awareness program aimed at the elderly, who are often targets for fraud.

For its part, the U.S. Postal Service conducted an eight-month investigation last year, and in October 2007 arrested 77 people as part of a global fraud crackdown that had intercepted more than $2.1 billion in counterfeit checks bound for the United States. The checks were intended for a number of different scams.

Again, not all mystery shopping jobs are scams. There are legitimate ads for jobs in newspapers, Web sites, and emails. The Federal Trade Commission (FTC) recommends visiting the Mystery Shopping Providers Association’s (MSPA) Web site for information on how to register to be a mystery shopper with an MSPA-member company, for a database of available jobs, and for additional information on the industry in general. MSPA has more than 180 member companies worldwide and has among its goals the aim of establishing professional standards and ethics for the industry and improving the industry’s image.

Meanwhile, here are some tips for knowing the difference between the real thing and a scam:

• When it’s legit, you get paid to visit a store, purchase a few things, and evaluate the experience. The hours are flexible, and you get to keep what you buy. You will never be asked to wire funds anywhere or to cash checks or money orders.

• A legitimate market research company will never ask you to pay an up-front fee in order to get information about a certification program, get a directory of mystery shopping companies, or to guarantee a mystery shopping job. A list of companies that hire shoppers is available for free, and legitimate jobs are on the Internet for free. Also, the FTC says companies that use mystery shoppers generally do not require certification.

• Mystery shopping jobs are not lucrative. They usually pay from $10 to $25. Shoppers are not promised hundreds of dollars.

• You’ve likely heard this catchy phrase before: If it sounds too good to be true, it usually is—even if it has been advertised on television or in the newspaper. Newspapers and TV stations generally do not check out individual advertisers unless they receive complaints. Some ads promise a money-back guarantee, but consumers often face difficulties in getting refunds. Usually it is impossible to contact the company or have them return your call.

If you think you have encountered a mystery shopping scam, file a complaint with your local consumer protection agency, the Better Business Bureau, your state attorney general, and/or the FTC.