A man walks into a post office with a $49,000 check. This isn't the beginning of a joke, but the start of a major scam.

"It wasn't quite the normal customer, said Melinda Martel, a customer service manager with the US Postal Service. "There was something about him that my staff picked up."

The man asked to buy several rolls of stamps and cash his check, so the clerk asked him for ID.

"He's insisting that he's in a hurry. He's insisting that he doesn't get questioned for ID," said Martel.

Postal employees were purposely trying to keep him there so they could get a picture of him and alert inspectors.

"He got nervous, waiting for so long for us to attend to him, and he wasn't getting his goods. And he got nervous, left the check, and walked out of the building," Martel explained.

The post office prevented this particular loss, but this suspect had been running an elaborate check fraud scheme for months.

"They use checks that are either bad or they're drafted from accounts that are closed and they use those checks to make purchases," said US Postal Inspector Troy Dickinson.

In this case it was stamps.

"He would buy 400 to 600 stamps at one point and that's unusual for someone to buy so many stamps," Dickinson explained.

He would sell them to anybody.

"To pawn stores, or people on the streets," said Dickinson. "He might say, 'I'm a small business owner and I just need money real quick. I just bought all these stamps and I'm willing to sell them to you for a discount.'"

This was not his first operation.

"This was one person who figured out how to take advantage of a situation. He wrote numerous checks. He got the post office for almost $40,000," Dickinson added.

The suspect was caught and convicted and is now serving a seven-year prison sentence.  Inspectors add that this case is an important reminder to always protect your personal information and checks.