Until 1923, Postma worked as a painter near Hilversum in Laren. He then moved to Amsterdam in the 1930s where he worked for art dealer Pieter de Boer and continued to develop his skills as a painter. Postma collaborated with other artists such as Carel Wilink and Pyke Koch, with whom he organized exhibitions.

Postma moved to Paris in 1939 where he worked as an artist until the German occupation. During the war he acted as an art agent and dealer.

Jean-François Lefranc, a well-connected French art dealer who facilitated sales for Nazi collectors, instigated the search for and seizure of the Schloss Collection by enlisting the help of both the Vichy government and the Nazi authorities. Once the Schloss Collection was seized and broken up, Lefranc received 22 paintings from it and sold them to a Dutch dealer known as Buitenweg. Postma consigned the painting by Giovanni Battista di l'Ortolano, Christ déposé de la croix (Schloss German no. 277/ French no. 303), with Galerie Claude, which offered it for sale at the Hôtel Drouot in Paris.

Also from the Schloss Collection, Postma sold in mid-1944 the Rubens work Paysage par un temps d'orage (Schloss German no. 271/ French no. 220) to Franz Rademacher, director of the Rhenisches Landesmuseum in Bonn. The American authorities were informed of the whereabouts of the painting and recovered it from Rademacher in September 1944.

After the war, Postma married Pieternella Wilhelmina (Lily) Bosman (1905-1966), former wife of the Jewish entrepreneur Oscar van Leer.

Sources

"Kor Postma". RKD-Netherlands Institute for Art History. Accessed 4 June 2021.

https://rkd.nl/nl/explore/artists/record?query=kor+postma&start=0

"Postma, Cornelius". Lost Art Internet Database. Stiftung Deutsches Zentrum Kulturgutverluste. Accesed 4 June 2021. https://www.lostart.de/Content/051_ProvenienzRaubkunst/DE/Beteiligte/P/Postma,%20Cornelius.html