After reviewing the records which Montias and others have uncovered, two facts become apparent. First, Vermeer’s paintings commanded relatively high prices when compared to many of his contemporaries. The price of six hundred livres that the baker thought reasonable for his painting compares favorably with the six hundred livres that Gerrit Dou (1613-1635) asked from de Monconys for his Woman in a Window, clearly also a painting with only one figure. Evidently, a painting by Vermeer had the same market value asa work by Dou, whom King Charles II of England had invited to become his court painter in 1660. Dou, one of Rembrandt’s prized students, commanded very high prices for his work throughout his career.
In addition, Vermeer apparently sold his paintings to a very few affluent clients who were capable of recognizing the extraordinary quality of his work, despite the fact that his fame was not nearly as widespread as other some Dutch masters of the time,including Dou or Gerrit Ter Borch. During his life, Vermeer’s fame did not generally reach much farther than nearby The Hague. Nonetheless, word of the artist’s talent was passed from one connoisseur to another in a relatively strict circle. Other than the six occasional but distinguished buyers of a single work, only Van Ruijven could possibly be called a patron. The Delft baker, Pieter Van Buyten, had purchased one painting from the artist while he was still alive and received another two after Vermeer’s early death as payment for a huge debt the artist had accumulated. Three paintings acquired in such circumstances probably don’t establish Van Buyten as a true patron, either.
Apparently, Vermeer’s only real patron was Pieter Van Ruijven. Although the exact nature of Vermeer’s relationship with him is subject to debate, it seems likely he had acquired at least some works directly from Vermeer. In fact, Van Ruijven’s son-in-law Jacob Dissius had in his possession twenty-one Vermeer’s at the time of his death. If we accept Montias’ estimate of the total number of Vermeer’s paintings to be from 44 to 54, this would mean that either Van Ruijven, or members of his family, had bought about one half of Vermeer’s entire artistic output.
Montias himself believes that “the relationship between Van Ruijven and Vermeer went clearly beyond the routine contacts of an artist with a client.” Van Ruijven lent Vermeer money. He witnessed the will of Vermeer’s sister Gertruy in her own house shortly before her death. More significantly, Van Ruijven’s wife, Maria Knuijt, left Vermeer a conditional bequest of five hundred guilders in her will.1 Such a testament was extremely unusual at the time.
However, the distinguished Vermeer specialist, Arthur Wheelock, has expressed reasonable doubts about the exact nature of their relationship. “The hypothesis that Van Ruijven was Vermeer’s patron, although appealing, should be cautiously approached, for no document specifies that Vermeer ever painted for Van Ruijven. Moreover, no source confirms that Van Ruijven himself had any Vermeer paintings in his possession. While Van Ruijven may have acquired painting from Vermeer, it seems unlikely that he assumed such an important a role in the artist’s life as Montias suggests. Should Van Ruijven had been Vermeer’s patron, one would expect that Balthasar de Monconys would have visited Van Ruijven himself in 1663, rather than the baker, Hendrick van Buyten, upon hearing that Vermeer had no paintings at home. Similarly, the Vermeer enthusiast Pieter Teding van Berckhout would also have made an effort to see the Van Ruijvens’ collection in 1669 on his two visits to Delft.” Wheelock further states: “While it is probable that some of the twenty Vermeer paintings listed in the inventory of 1683 ( the inventory taken after the death of van Ruijven’s daughter) came from Van Ruijven, others may have been acquired by {his daughter} Magdalena, Jacob Dissius (his son-in-law), or his (Jacob’s) father, Abraham Jacobz Dissius, at a sale of twenty-six paintings from Vermeer’s estate held at the Saint Luke’s Guild Hall on 15 May, 1677.” 2
While the precise relationship between Vermeer and van Ruijven may never be known, it is evident that the Van Ruijven family held Vermeer’s work in high regards, having, at one time or the other, assembled a significant part of the master’s oeuvre.
http://www.essentialvermeer.com/clients_patrons/vermeer’s_clients_and_patrons.html


