The phony checks look real to you, and even look real to your bank. If you get one of these checks and deposit the money, it shows up in your account just like normal, but that's when the trouble begins.

The scam takes advantage of the fact that just because a check supposedly clears and the bank posts the money in your account, it doesn't mean the bank can't later take that money back if it finds out the check was a counterfeit.

That's just what happened to Jill Parker, a pharmaceutical company manager in Richmond, Virginia.

Using the popular Craigsist web site to rent an apartment she owned in Chicago, she was contacted by someone moving from London.

"He was going to send me a check for $25,000. I was to deduct what he owed me for the first months rent and the security deposit and then I was to wire the balance back to his agent, who was handling his furnishing," Parker said.

She took the check to her bank and called a few days later to see if it had cleared.

"He said the money is there, it cleared, it's there," she said.

So, as agreed, she wired the remainder, $21,000, thinking she was ahead by $4,000.

"Everything went fine until about a week later," Parker said.

The bank informed her the check was no good, and had been returned not paid. And she, not the bank, was out the money.

"We felt terrible to find out that we had been victimized," she said. "You feel very vulnerable."

Tens of thousands of Americans are asking the same questions, many of them targeted online through Craigslist and eBay.

"They send you a check and they over pay, and then they ask you to refund the difference and so it's a very quick transaction," said Postmaster General Jack Potter.

U.S. Postal Service officials say they have seized more than $2 billion worth of high quality counterfeit checks, coming from Nigeria, England, the Netherlands and Canada.

But many more phonies are getting through.

"Under federal law, banks are required to make money available according to a certain schedule," said American Bankers Association spokeswoman Nedda Feddis. "Certain funds, for example, have to be available on the day after deposit. And the fraudsters are taking advantage of that rule."

There have been tragic consequences.

Chris Soens, suffering from health problems, got good news in the mail, she won $90,000 in a supposed European lottery.

She wired back $40,000 for what she was told were fees and taxes.

When the original lottery check was discovered to be a phony, the bank told her she had to repay the entire amount.

Her sister Rebecca says it led to Chris' suicide.

"I think she was devastated," Rebecca said. "I think she was plunged into depths of despair knowing that everything that she had was gone."