Bill Pallot had an unparalleled passion for 18th-century French chairs that he turned into a lucrative career consulting with museums, galleries, collectors and the Palace of Versailles.

He became a fixture in Parisian society and a celebrity in the art world, until he was felled by a former student who had become so steeped in antiques that he could — literally — taste a fake.

At the height of his powers, Mr. Pallot’s expertise and assurances of authenticity had helped convince French experts to designate multiple items as national treasures. He also used his renown to dupe deep-pocketed buyers, including Prince Abdullah bin Khalifa Al-Thani of Qatar, into believing they were purchasing genuine pieces of royal history.

He attested to the authenticity of seating said to have belonged to Marie Antoinette and to the mistress of Louis XV, Madame du Barry.

People believed so fully in Mr. Pallot because almost 40 years ago he wrote what was long considered the book on the topic: “The Art of the Chair in 18th Century France,” which includes a preface by his friend, the antique enthusiast and fashion designer Karl Lagerfeld.

Now, Mr. Pallot is perhaps best known for using his knowledge of art history to hoodwink some of the most esteemed antique experts and buyers.

On Tuesday, after years of investigations by the French police, Mr. Pallot and five others said to be involved in a scheme to unload fakes onto unsuspecting buyers attended the first day of a criminal trial in Pontoise, near Paris, where they stand accused of trafficking in counterfeit antique furniture.

In 2016, the French culture ministry issued a statement saying that the police were investigating the authenticity of pieces of furniture valued at 2.7 million euros (about $2.9 million), including two Louis XV chairs, purchased by the Palace of Versailles. That inquiry led to the conclusion they were not authentic and to the arrest of Mr. Pallot the same year. In 2017, the scandal also changed how the French authorities authenticated antiques.

But doubts about Mr. Pallot had begun to surface years before, most notably for his fellow antique dealer and former student, Charles Hooreman, who shared his concerns with Mr. Pallot, as well as with buyers and the French authorities.

In 2018, Mr. Hooreman told Vanity Fair that he had considered Mr. Pallot his “hero” after attending his art history lectures at the Sorbonne. He later entered the same profession as his teacher, but he became suspicious of his mentor based on conversations with a buyer and about the quantity of antiques surfacing.

As far back as 2012, Mr. Hooreman said he had seen two folding benches that were being touted as having belonged to Princess Louise Élisabeth, the eldest daughter of King Louis XV. He felt compelled to test them.

“I licked the chair and voilà. I could taste the fraud,” he told Vanity Fair.

Familiar with the methods used by master craftsmen for restoration, he recognized a trick used by a woodworker whom Mr. Pallot favored, Bruno Desnoues. Mr. Desnoues used melted licorice to give new wood an old feel.

Mr. Desnoues is also on trial now and has admitted his role in the scheme. Mr. Pallot himself has widely admitted complicity but denies there are as many fakes as Mr. Hooreman has claimed.

An investigative judge in France noted in a previous proceeding that a smiling Mr. Pallot had told the court about the origins of the deception: He and Mr. Desnoues had become curious about whether they could make a good fake one day while the craftsman was restoring authentic antiques, according to Le Monde.