When does a check clear? Who has the authority to determine when it is paid? This is not a matter of idle concern to me, because I buy and sell on Ebay, and I've taken checks, and when I have, I've told people that I'd send out the item as soon as the check "cleared."

But what does "cleared" mean? Naturally, I always assumed that when the bank says the check has cleared and funds are available, that it means what it says.

I didn't know this before, but I was recently shocked to learn that this is not the case. Not if the check turns out to be phony:

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.

Common sense suggests that a check cannot clear and then later "unclear."

Unfortunately, the banking laws say otherwise:

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."

I also found this account of a customer who was told that he could not rely on what his own bank told him:

My family placed an ad to rent out an apartment. A person from out-of-town answered and sent a check after some phone calls and emails. Now we realize that it was a scam! In any case, the "renter" send a check for $3995 and asked us to send $2000 of it to a "furniture rental company" for him.

I went to my JP Morgan Chase bank and asked, how do I know the check is good. The teller told me that if it clears then it is good. So I deposit the check on 4/12 and was told by the teller and the info on the deposit slip that it will clear on 4/18. Then I called the bank 3 time and asked 3 different people how do I know a check is good. All of them said if it clears on 4/18 then it is good.

So on 4/18, the full $3995 was credited to my account, we sent a western union to the "furniture rental company". Then on 4/20, the bank retracted the $3995 amount and said it was a bad check. Obviously, by then, the western union money was claimed already and we never heard from the "renter" again.

Now, the part about the bank is that I asked 4 different employees how I can tell the check is good. All told me if the check clears on 4/18 then it is good. So my decisions were based on the bank's advice. I feel they are responsible for the loss. Am I right? Is there anything I can do? The branch manager said I have no recourse. So do I need to go to court, write to FDIC? What are my chances?

Even if "banks are required to make money available according to a certain schedule" as the spokesman says, I think banks should be required to disclose whether a check has actually cleared. If they falsely state that a check has cleared when it has not, it seems to me that they should be held liable, not the customer.

None of this is to say that the individuals in the above scenarios weren't lacking in common sense. I certainly would not put a large foreign check through my account and then send back the difference as these people did -- but then, I wouldn't imagine myself to have won the European Lottery because an email said so, either. Some people are more gullible than others. But isn't that why they need to be able to rely on what their banks tell them? Assuming these factual scenarios are accurate, I think the conduct of the banks in telling them checks have cleared when they haven't is indefensible.

The banks are in a superior position, and customers should have the right to rely on their assurances.

It would be interesting to see what would happen if one of these cases went to court.

MORE: As a news subject, the "cleared but not cleared" check story is starting to attract more attention:

NEW YORK (Reuters) - Patricia Soens paid more than $400,000 in processing and other fees to claim a European lottery prize she was told she had won. After realizing she had been conned, she killed herself.

Soens, 56, of Louisiana, fell victim to one of a growing number of scams that use fake checks and the promise of easy money to lure people.

The schemes, which range from fake lotteries to work-at-home offers, exploit the fact that banks have to provide funds from checks, money orders and other financial instruments before they actually clear.