PARIS — “It’s a real kerplunk.” José Luis Celada smiled with satisfaction as he closed the rear passenger door on his 1951 Alfa Romeo 6C 2500 S. “That’s a really strong door,” he added. “All four of them are like that.”

Mr. Celada, 65, a collector based in Buenos Aires, had just bought the immaculate maroon Alfa saloon for 143,750 euros, or about $177,000, at an RM Sotheby’s auction in Paris on Feb. 7. The car, thought to be one of just 17 surviving examples of the model, sold for far less than the presale estimate of €175,000.

The result and the buyer were both typical of the trading in classic automobiles in Paris during the 43rd annual Retromobile fair and its accompanying auctions.

Demand has cooled considerably since 2013, when prices soared as much as 62 percent for collectible Ferraris, for instance, making classic cars the hottest “passion investment.” Equivalent Ferraris increased 2.5 percent in value in 2017, according to an index compiled by Historic Auto Group International, based in London.