BRENTWOOD — The former owner of the now-shuttered Nashua classic car consignment business Dusty Old Cars was convicted of theft Wednesday and now faces the possibility of up to 15 years in state prison.

It took a Rockingham County jury about 40 minutes to find Stephan Condodemetraky guilty of a felony charge of theft by unauthorized taking.

Condodemetraky, 50, of Bedford, was accused of keeping $5,978 that prosecutors say was owed to customer Wesley Ridlon when his company signed a contract to sell the 86-year-old man’s 1941 Buick.

Prosecutors said the vehicle was sold through consignment for $21,000 in 2015, but Ridlon received just $10,000. He was later given another $500, but was expecting thousands more and filed a lawsuit to try to recover the money.

An invoice that Ridlon later received showed he was charged a $5,978 “marketing fee” that he never authorized, Assistant Attorney General John Garrigan said.

“So what is this marketing fee? It’s bogus. It’s a con job,” he told the jury during closing arguments Wednesday afternoon.

Condodemetraky is scheduled to be sentenced on April 29. He faces 7½ to 15 years in prison and a $4,000 fine.

The two-day trial is the first of several that Condodemetraky is expected to face over the next year.

Condodemetraky faces more than three dozen charges in Hillsborough, Merrimack, and Rockingham counties accusing him of sales fraud and other crimes related to the business, which filed for bankruptcy.

Garrigan said three more trials are expected in Rockingham County with an additional trial in both Hillsborough and Merrimack counties.

Judge Andrew Schulman suggested the possibility of both sides trying to settle the other cases in the wake of Wednesday’s conviction.

Condodemetraky chose not to testify in his own defense. Defense attorney Bruce Kenna called no defense witnesses, but argued that his client wasn’t a thief and had attempted to pay any debts with consigners.

Kenna said prosecutors offered no evidence to prove that Condodemetraky “personally ever stole anybody’s money or that he ever took anybody’s money that he didn’t have a right to or that his company didn’t have a right to.”

He added, “He had nothing to do with the purchase of this car other than his company did it.”

Garrigan disagreed.

“He used his company to pay his personal expenses so any money kept by the company benefited him directly,” he said.

Kenna suggested that the $5,978 fee could have simply been a mistake.

But prosecutors didn’t buy it.

Garrigan said Ridlon made repeated attempts to get the money he felt he was owed.

“Month after month the defendant said the money was coming and month after month after month the money never came,” he said.