At the International Property Rights Center in Washington D.C., director Lev Kubiak said selling counterfeit items was a "very, very lucrative business."

"I think most Americans have no idea" just how much of what they're buying is actually fake, Kubiak said.

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The Federal Trade Commission says it's been cracking down in recent weeks, sending cease-and-desist letters to companies that it says are making false or uncertified claims.

Kubiak showed ABC News a room containing fake items the center had seized from store shelves and Internet sites across the U.S., including the U.S.-made language-learning tool Rosetta Stone.

"This one came from China," Kubiak said, pointing out some tell-tale signs: misspelled words such as "guaranted" and "Piece" Corps, rather than Peace. The counterfeiters even forged the Rosetta Stone website.

At the Rosetta Stone factory in Harrisonburg, Virginia, Rula Abu-Taleb, a product-control clerk, said counterfeiters took jobs away.

"It's sad for us. … There's someone out there that's making their own money off of our hard work," she said about the fake products.

Rosetta Stone said the secret to its success was its U.S. workers.

"I love the creativity that comes out of our company," said Steve Swad, president and CEO of Rosetta Stone. "I love the creativity that comes out of the U.S.. … I look at counterfeiting as an obstacle that we're going to overcome."

Rosetta Stone is fighting back against counterfeiters, though, investing in smart technology to scour the Internet looking for crooks.

"I think if we took this process overseas, it would be one job among many, with people that didn't care half as much as these folks care," said director of operations David Litherland. "It just wouldn't be the same quality of product shipped out to our customers."

ABC News

ABC News' Eric Noll contributed to this story.