The biggest problems consumers are facing in the U.S. marketplace include some familiar suspects, such as shady used-car salesmen, fake mortgage-rescue deals, online scams and soaring credit-card rates and fees.

A survey of state and local consumer agencies around the country conducted by the Consumer Federation of America found that the top complaints last year related to autos, home contractors, loans, retail disputes and utility or cable providers. Other complaints related to faulty household goods and repairs for big-ticket items, fraudulent Web solicitations or Internet sales, telemarketing and home solicitations, and landlord-tenant disputes.

"In economic hard times, consumers are even more vulnerable to phony promises to help them financially or money-making schemes," says Susan Grant, CFA's director of consumer protection.

In one familiar incident related to the housing crisis that is crushing consumers' wealth and contributing to their debt burdens, a Georgia company charged an initial $400 to $500 to help consumers whose homes were in danger of foreclosure, plus other fees for appraisals, title searches, processing and refinancing. It reached out to customers through Web advertisements and direct mailings.

However, according to the governor's office of consumer affairs, none of the services offered were ever provided and most of those who paid the steep fees lost their homes. The agency is still investigating the company, which CFA did not name.

Bob Harris, who manages the Office of Consumer Protection in Washington, D.C., says state and local agencies have also had to develop "expertise to deal with the evolving nature of fraud, especially online."

For instance, in one online scam, a Florida man was deciding whether to purchase a vehicle from an online seller. Before sending a payment, he checked with the Better Business Bureau to see whether there were any complaints, but found that the dealer had a good reputation. He then wired a $4,000 deposit, but never heard back from the seller or received the merchandise. An investigation by the Consumer Fraud Unit in Orange County, Fla., found that the crook posed as a legitimate dealer by copying its Web site, then picked up the payment using a phony passport.

It can be difficult for even the savviest consumer to thwart rip-offs, but here are some useful tips to avoid becoming a victim:

1. Beware of "danger signs" of fraudulent schemes

. Don't get pressured into a deal you're not comfortable with. If a merchant tries too hard to convince you about the value of the company's product or service, what it's presenting is probably not true. Merchants also shouldn't have to play to your sympathies or use scare tactics to force you to purchase or agree to something immediately.

Avoid any purchases that require a wire transfer and merchants that reach out to you though mailings, telemarketing and online ads promising deals that seem too good to be true. Those struggling with mortgages or facing foreclosure are particularly vulnerable to such offerings and should call

government agencies

or

consumer advocates

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for help before discussing the situation with someone who wants to profit from their misfortune.

2. Check into merchants and their products

. Dubious merchants can be found online, behind the sales counter or in the passenger's seat during a test drive.

Though it didn't help the unlucky Florida consumer, anyone buying expensive items should investigate the seller's history with the

Better Business Bureau

and consult a local consumer agency. Ask for references when applicable, and make sure contractors and other professionals are licensed. When buying from online outlets, make sure there's a customer-service line and legitimate address to call or locate the business if trouble arises.

Make sure any goods you're buying are not part of a

product-safety recall

. At auto dealerships, request all documentation related to the vehicle you're interested in, and question the dealer about high mileage on "new" cars.

3. Use a credit card for items purchased online or those that need to be delivered

. That way, you can dispute charges if you don't receive what you were promised. Of course, make sure you can afford the item, otherwise you'll be paying interest or late fees as well.

4. Ask to make a down payment instead of paying in full, up front

. Consumers often complain that home-improvement work and other services were not conducted in the manner promised. But once you've paid the tab, it's more difficult to coax subpar contractors to clean up their messes or finish what they started.

5. Get everything in writing and understand everything you sign

. Service estimates, as well as any other agreements with merchants, lenders, landlords and roommates should always be in ink on paper -- but never sign an agreement you don't fully understand or aren't comfortable with. Before signing any document that is written up by someone else, read it thoroughly or consult a lawyer, accountant or other expert to comb through the details.