Webster's Revised Unabridged Dictionary · 1913

deaccession

deaccession , transitive verb

[de- + accession. Ca. 1970.]

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.

to sell (artwork); -- used of sales of art by museums.
sell

Also: de-accession