“It isn’t an app built for billionaires, but for everyone interested in art,” Mr. Resch said. “This isn’t a selling platform. Our aim is to provide access to the art market and democratize it and hopefully enlarge it.”

Magnus now has more than eight million gallery and auction prices compiled worldwide since 2013 by more than 200 of the app’s preliminary “trusted users.” Mr. Resch declined to specify how many of these are “primary” market prices — the first time a work has been sold — and how many are “secondary” resales at auction, for which data is available at subscription websites like artnet.com and artprice.com.

Mr. Resch said prices given by a gallery had to be corroborated by other trusted users before being added to the database. The app also includes an interactive map with information on gallery openings and shows, as well as museum exhibitions.

To see whether Magnus works as advertised, Lisa Schiff, a New York art adviser, took a smartphone loaded with the app to the Matthew Marks and Marianne Boesky galleries in Chelsea this week.

Mr. Marks was exhibiting vintage photographs by Ellsworth Kelly, Ms. Boesky new paintings by the Johannesburg artist Serge Alain Nitegeka, neither of which could be regarded as “mainstream” shows. Ms. Schiff said the app recognized and supplied current gallery prices and historic market data for all but one of the works she photographed (people often take photographs of artworks with phones in commercial galleries, and the practice is generally permitted, provided the image is for personal use.)

Ms. Schiff, who has been an adviser for 15 years and who does not know Mr. Resch, described the app as a “game changer,” though she noted she could not find any data on private resales by dealers.