Over the next few weeks, NBC News purchased three pieces of sports memorabilia advertised online by Ingber and Perullo: separate baseballs purportedly signed by Willie Mays, Mickey Mantle and Joe DiMaggio. The total cost: $1,093.

The balls were all deemed not genuine by Spence's company. A second authenticator, PSA, also determined that they were inauthentic.

A total of 18 items had now been tested, and Perullo was batting zero for 18.

The dealer speaks

The name Ralph Perullo is well known among longtime collectors of sports memorabilia. In 1999, the Charlotte businessman attracted a swirl of media attention when he bought thousands of items from DiMaggio's personal collection shortly after the Yankee Clipper's death.

"I'm still pinching myself," Perullo, a New Jersey native who grew up idolizing DiMaggio, was quoted as saying at the time. "This brings back so many memories."

Perullo was reported to have snapped up thousands of bats, balls, jerseys and photographs — all autographed by the legendary slugger. The multimillion-dollar deal was orchestrated by DiMaggio's longtime attorney and friend, Morris Engelberg.

"I was impressed with Ralph the first time we met," Engelberg said at the time. "He said he wouldn't do anything to hurt Joe's reputation. I trusted him."

Perullo immediately put the items on the market in that summer of 1999.

Nearly two decades later, Perullo was still trading on the DiMaggio name. The website for his company, Memorabilia Center, describes it as the "home of exclusive Joe DiMaggio autographed memorabilia." He also sells items through an ebay account called joedimaggioestate.

Reached by phone, Perullo said he'd be glad to sit for an interview with NBC News to discuss sports memorabilia and the prevalence of fraud in the industry.

Ralph Perullo assesses signatures on two baseballs. NBC News

Perullo showed up to the interview in a long-sleeve purple shirt and khaki pants, holding a medium-sized cup of coffee filled to the top. Perullo, 78, walked with an unsteady gait. Within minutes of stepping into a hotel room, he spilled his entire cup of coffee on the floor.

It was not the kind of first impression that presented as master conman.

During a wide-ranging interview that lasted nearly 90 minutes, Perullo said he bought the DiMaggio collection for roughly $3 million. He conceded that he unloaded nearly all of it within five years and then began buying various pieces of sports memorabilia from other sources.

Perullo said he was concerned about fraud "from day one — because it was the easiest thing in the world to do." As a result, he said he did his best to "follow the history" of an item.

Perullo agreed to offer his assessment of two balls purchased by NBC News, unaware that he had sold them. It took Perullo less than five seconds to declare that the Willie Mays signature was fake.

"Willie wouldn't sign that ball," Perullo said. "It's not a Major League baseball."

It was an extraordinary moment: a sports memorabilia dealer admitting that an item he sold was a fake.

But Perullo remained calm when told the ball came from his company. And he offered a matter-of-fact reply when asked how he came to sell a fraudulent item.

"That's a good question," he said. "You got me."

Perullo also stayed composed when told that all 11 of the items tested from Kerr's collection were determined to be inauthentic. "Why didn't they call me?" he said.

Sullivan had spoken to Perullo. She had also received the email from him threatening to a put a lien on Kerr's estate. Perullo was adamant that he sent no such email. But after he was presented with the email itself, Perullo suggested that someone else must have written it.

"This is embarrassing," he said. "...I would never write a letter like that. Never."

Perullo acknowledged that it's possible some bad items had slipped into his collection. But he was adamant that the bulk of his merchandise was legit. He said he even had six authenticators on staff to guarantee that he wasn't selling bogus goods.

"We wouldn't be in business for 20 years if that's what we did," Perullo said.

But the Kerr case is not unique. NBC News tracked down two other customers who bought items from Perullo that were later determined to be inauthentic. They both told similar stories of buying items through the joedimaggioestate eBay account and later taking them to authenticators who determined the items were not genuine.

One of the men said he paid more than $1,000 on Perullo merchandise but got the money back. The other said he was still out roughly $13,000. Both said Ingber responded with threats when they called for refunds.

"I am really scared of these people," said one of the men. "I don't know what they're capable of."

Like the other customer, he spoke on the condition of anonymity because he fears Ingber's wrath.

"I used to sell memorabilia," the man said. "I knew to stay away from people like this, but I got suckered in by the Joe DiMaggio name."

In emails to NBC News, Ingber said he did "not recall" making any threats as described by the two former customers. Ingber insisted that he had an "excellent relationship with 99 percent" of his customers. He also questioned the credibility of third-party authenticators such as JSA and PSA.

As for the Kerr case, Ingber flatly dismissed the claims by the Sullivans.

"This Chuck Kerr matter is a nonstory," Ingber wrote. "What it looks like to me is some gal who inherited her father's estate and did not want to honor his payment obligations."

Ingber sent NBC News emails from the Sullivans that he said showed they were the ones making the threats. In one of the messages, Sullivan said the family was going to consider "getting the proper authorities involved in investigating such fraud unless we can come to a reasonable accommodation."

Ingber also denied knowingly selling pieces of sports memorabilia that weren't genuine.

"If I had any definitive proof that items Mr. Perullo was putting out for us to sell was not authentic, I would quit," Ingber wrote.

But it's not just former customers who cast Perullo's business practices in a poor light.

Debbie Schoenack worked as an office manager at a Minneapolis-based sports memorabilia company that Perullo co-operated in the mid-2000s.

After working with him for two years, Schoenack came to believe that Perullo was "just plain crooked all the way around."

"Sometimes there would be an item that didn't come with a certificate (of authenticity) and all of a sudden he would just raise his magic wand and one would appear," Schoenack recalled. "And I thought, 'That's odd.'"

She said customers often called to complained about the items they had purchased. "His m.o. was, 'They don't know. What do they know? They can't prove anything.'"

Schoenack worked for the now-defunct company, Collectors Source, from 1997 to 2009. Perullo was brought on to help run the business around 2007.

She described her time working for the company as an eye-opening experience — for all the wrong reasons.

"It's just an unethical business all the way around," Schoenack said. "I think if you really had the time and really delved into this, you could find a lot of Ralph Perullos out there."

He couldn't believe his eyes

It took nine months, but earlier this summer the Sullivans finally finished inventorying what they could of Kerr's purchases from Perullo. The tally came to $135,537.40, according to credit card statements shared with NBC News. But the Sullivans said they don't have access to all of Kerr's statements and believe that the total amount easily exceeds $150,000.

"He really honestly thought that this was going to enable us to put our children through college," Sullivan said. "That these items were going to take care of us in the long run."

"This was his retirement plan," Brian Sullivan added. "This was his 401K."

The Sullivans are still hoping to find a way to recoup more of Kerr's money but they're not especially hopeful. They said they wanted to speak out to warn others of the pitfalls in buying sports memorabilia.

"Do your due diligence," Sullivan said, "especially when buying something online."

In March, the Sullivans returned to Perullo four items that the credit card company had refunded. They made sure to include the JSA letters saying the items were found to be inauthentic.

The Sullivans also dropped into the package a note that read in part: "Please do not try to dupe another person as you duped Chuck for years."

One month later, Brian Sullivan went online and pulled up Perullo's Memorabilia Center website. He said he couldn't believe his eyes at first. But there it was: a listing for one of the items they had returned.

A Derek Jeter-signed bat — back on sale — at a price of $695.