Quentin Metsys, The Moneylender and His Wife (detail), 1514.
Artprice is particularly useful for tracking trends in the market geographically and for specific artists. Coverage from 1983.
artnet is useful for finding out estimates and realised prices for specific lots, or filling in blanks in your knowledge of specific lots or sales. Coverage from 1985. It has much better coverage of decorative arts and design than Artprice.
Invaluable specialises in sales information for decorative art objects.
Benezit Dictionary of Artists includes selected auction records for prominent artists. Useful for seeing sample prices from before the period covered by Artprice and artnet.
For auction houses' own online records, see:
For details of more UK auctioneers, see the Society of Fine Art Auctioneers and Valuers.