The Collaborative International Dictionary of English v.0.48:

deaccession \deaccession\, de-accession \de-accession\v. t. [de-
   + accession. Ca. 1970.]
   to sell (artwork); -- used of sales of art by museums.

   Note: The word appears to have been coined as a euphemism to
         avoid the negative connotations of the word "sell",
         particularly for works donated by benefactors. The
         practise is justified as a means of acquiring funds for
         purchase of more valuable artworks.
         [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

deaccession
    v 1: sell (art works) from a collection, especially in order to
         raise money for the purchase of other art works; "The
         museum deaccessioned several important works of this
         painter"