I think what we do here is similar to discussions in papers (scientific) with well established and accepted quoting, paraphrasing and summarizing.
Even well known commercial websites allow quoting and images from publications to support their claims of authenticity. It happens too often to be an aberration and websites management must be aware of the practice.
This example follows the format of an exhibition catalog but lists the auction house as the author as per the guidelines for works published by an organization, corporation, or institution.
Sotheby's New York. Important Jewels. New York: Sotheby's, 2007. Auction catalog.
In short, I suppose everything is fine if the source is quoted.