Check schemes are spreading across the United States as scamsters exploit the popularity of online auction, dating and social-networking sites to find victims.

The Postal Inspection Service has launched a TV and print campaign — its largest-ever anti-fraud drive — to alert consumers. Overseas investigators, working with U.S. postal inspectors, have arrested 77 people this year and intercepted $2.1 billion in counterfeit checks headed for the U.S. And the Federal Trade Commission sued two Canadian companies in October, accusing them of using fake checks to bilk U.S. consumers.

Even as consumers reduce their use of checks, the creation of fake checks is booming. Fraudsters use them to pay for goods online or to convince people that they've won sweepstakes prizes.

If customers lose money because of check fraud, their banks won't bail them out. But if those customers can't repay, the banks get stuck with the losses.

No one knows how much money customers have lost from bogus checks. But last year, banks alone lost $271 million from fake checks — a 160 percent jump from three years earlier, according to the American Bankers Association.

In one common scam, fraudsters contact sellers at online auction sites, saying they want to buy. They say they'll send a check for more than the sale price, explaining that the excess money must be wired to a third party who will handle the shipping.

The fraudsters often don't bother to claim the item. They just want consumers to wire money to them quickly, before the checks bounce, said Susan Grant, director of the National Consumers League's fraud center.

Generally, banks must make funds available to their customers one to five days after a check is deposited — even if the check has yet to clear. But because it could take weeks for a check to actually clear, the victim might not find out that the check is a fake until it's too late.