For that $30,000, Espinoza had agreed to sell a slew of bitcoins, the almost unregulated virtual currency, which Arias’s character said he would exchange for stolen Russian credit-card numbers.

The sting was designed to catch Espinoza, then 30 of Miami, laundering money. Florida law prohibits using financial transactions to “promote” illicit activity, such as, in this case, credit-card fraud.

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Ultimately, Arias arrested Espinoza on three felony counts of money laundering, capping a three-month investigation in 2014 into South Florida’s exchange of computerized money.

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But a Florida circuit-court judge ruled Monday that bitcoin is not money at all. And if you don’t have money, you can’t exactly launder it.

In an eight-page opinion made available by the Miami Herald, Judge Teresa Pooler acknowledged that she could not explain what bitcoin is. “Nothing in our frame of references allows us to accurately define or describe Bitcoin,” she wrote.

But she was able to say what bitcoin isn’t: It’s not money.

“Bitcoin may have some attributes in common with what we commonly refer to as money,” she wrote. It can be “exchanged for items of value,” for example, but unlike money, not always. It’s accepted by some merchants and service providers but unlike money, not by all of them. Unlike money, bitcoin’s value fluctuates wildly. “With such volatility, they have a limited ability to act as a store of value, another important attribute of money.”

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The charges were dismissed.

It was only the latest chapter in a long and as yet unresolved argument about where virtual currency, most prominently, bitcoin, fits within a regulatory and legal framework that never anticipated its rise.

Bitcoin is a decentralized “virtual currency” with fluctuating value designed in 2009 as a way to transact business, sometimes illegal, without the constraints or scrutiny attached to real money.

Once created, bitcoins can be bought and sold online. Some retailers, including Amazon.com, Target and Microsoft, accept bitcoins or work with financial services firms to convert the coins into cash.

And all of that has left lawmakers flummoxed about how to approach the currency.

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“Attempts to categorize Bitcoin into existing regulatory constructs have proven to be difficult,” wrote Kevin V. Tu and Michael Meredith in the Washington Law Review.

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The Internal Revenue Service in 2014 declared that bitcoin would be taxed as physical property. If you sell it and make a profit, it’s taxable as if it were a house or a share of stock. Last year, the Commodities and Futures Trading Commission defined virtual currencies as commodities for regulatory purposes.

Despite its increasingly common use, many see bitcoin as inherently dirty, used when people have something to conceal. That is what Arias and Secret Service special agent Gregory Ponzi thought when they began their investigation in 2013.

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“There is unquestionably no evidence that the Defendant did anything wrong, other than sell his Bitcoin to an investigator who wanted to make a case,” Pooler wrote.

On localbitcoins.com, Arias and Ponzi found a user named “Michelhack.” He asked potential buyers to contact him anytime, day or night. He liked to meet in public places such as coffee shops and malls, he wrote on his online profile, and buyers had to pay in cash in person.

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Arias and Ponzi homed in. A man with 24-hour availability operating solely in bitcoin and cash was a sign of someone up to no good, they said. Arias on Dec. 4, 2013, asked for a trade: $500 for 40 percent of a bitcoin.

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The pair met at a Nespresso Cafe in Miami Beach. Espinoza took a liking to Arias’s character. The two sat and chatted after the sale. He told Arias about the bitcoin trade. He said he bought the coins for 10 percent below market value and sold at a 5 percent margin.

On his $500 sale, Espinoza made $83.67, he said.

A week later, Arias texted Espinoza again. He wanted $1,000 in bitcoin, and they met at a Miami Haagen-Dazs.

Over frozen treats, Arias told Espinoza that he used the bitcoin to buy stolen Russian credit-card numbers. During the next transaction, he offered to pay with those instead of cash.

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“I’ll think about it,” Espinoza replied. He and Arias did not speak for three weeks.

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A flurry of texts came Jan. 30, 2014. Arias wanted $500 worth of bitcoins. The two made the trade electronically.

Then came the detective’s big ask: $30,000 in bitcoins.

They met at a hotel Feb. 6, talking briefly in the lobby before heading up to the room Arias and Ponzi had bugged. There, the exchange happened, and Espinoza’s arrest for money laundering.

In front of Pooler, and for the next two years, Espinoza said he had not broken any laws. He was trading in bitcoin, not money, so how could he be charged with laundering money? That Arias wanted to use the virtual currency for illicit activity was none of Espinoza’s concern, his attorneys argued.

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Defense attorney Frank Andrew Prieto held up a 1966 U.S. quarter in court and asked expert witness Charles Evans, a Barry University economist, “Is bitcoin an actual coin?”

“In a sense of a physical piece of base metal?” said Evans, who was paid $3,000 in bitcoins for his appearance in court. “No.”