For decades, art advisers were a small club of professionals who personally helped build collections for clients, using their scholarship and connoisseurship. Their role was to consult and offer expertise, rarely to make deals.

But the rapidly changing art market — characterized by soaring prices, high fees and a host of wealthy new buyers from Wall Street and abroad — has prompted scores of new players to jump into the pool, from young art-world arrivistes to former auction-house executives with an abundance of expertise and connections.  The demand for advisers has grown in part because newly rich collectors need help navigating an increasingly transactional, famously opaque art market.

And for these buyers, advisers — once considered largely messengers between collectors and sellers — are increasingly empowered at auctions, galleries and art fairs. “We’re seeing them become more the negotiator than they traditionally might have been,” said Lisa Dennison, the chairwoman of Sotheby’s North and South America.  The best are wooed with elegant dinners, early notice of forthcoming offerings and private showings. “Ten years ago we didn’t watch them closely but now we are courting the most important ones assiduously,” said Victoria Siddall, the director of the Frieze art fairs in New York and London.