“They have connections to more money than any of the galleries anyway,” Mr. Ferrante said, adding that he would rather share his proceeds with his models than with a broker. “By turning the model into an art dealer, we’re cutting out the middleman.”

For Mr. Ferrante, the transactions are a welcome alternative to the established art world, in which, he said, dealer representation is increasingly hard to come by, and galleries often take as much as half of an artist’s earnings.

Such exchanges, historians and local artists said, reflected a hidden but not unprecedented intersection of mainstream art and high-end prostitution, extending from New York to Moscow and beyond.

The concept of men commissioning portraits of their mistresses is far from unusual, said Andrew Lear, a former classics professor at New York University and founder and president of Shady Ladies Tours, which offers tours in the Metropolitan Museum of Art that focus on the role of sex and sexuality in art. Courtesans abound in artwork as far back as the 18th century, Mr. Lear said.

One example on display at the Met, he said, is Thomas Gainsborough’s portrait of Grace Dalrymple Elliott from the 1700s. The painting, according to the Met’s website, “was apparently commissioned by her lover, the first marquis of Cholmondeley.”

Though he had not heard of a modern-day example before learning of Mr. Ferrante’s work, Mr. Lear said the transactions provided several parallels with the past. “It’s interesting that maybe things have changed less than we think,” he said.