Two observations:
1. We must not forget that the national museums are a great number of works of modern art for free - in Europe they are often transferred as payment for inheritance tax the heirs of the artist or collector, by the United States has the ability to pay income tax by transferring to a museum of his work; in anywhere in the world the artist willingly submits them to the gift in major museums.
2. Unlike the "big" museums, which rarely sell imeyushiesya in the collection of things (actually, the state museums did not have the right to do so, they must seek permission from the government or parliament), "small" private museums often sell the purchased several decades ago, the product Artists, meanwhile, lost its "relevance"
I remember that I tried to draw your attention to the origin of Komar and Melamid, sold at auction in the autumn Mac Dougall, to quote him:
Provenance: Ludwig Museum, Vienna, Austria.
Private collection, USA
and leave you to draw conclusions themselves (in general, in auction catalogs the most interesting - not images).