It is no surprise to most people that the value of paintings by famous painters depends at least as much on their authenticity as the quality of the work. An average work by a master is likely to sell for more than a masterpiece by an average painter.
This was clear in 2007 when a painting thought not to be a Rembrandt by most experts was estimated at £1,500. It sold for £2.2m to a collector who disagreed.
However, some 2009 sales have reminded us that artworks can gain value much like memorabilia - that is, if they are owned by or associated with celebrities.
In February, a number of items which belonged to the great fashion designer Yves Saint Laurent and his partner far exceeded their estimates.
In some cases, such as a Henri Matisse painting which sold for €32.1m, this may have due to the sheer desirability of Matisse's work. But other sales were more eye-opening.
A leather armchair designed by Eileen Gray, sold for €21.9m - and experts have speculated that Yves Saint Laurent's fame was responsible for this amount, rather than the item itself.
The auction brought applause and gasps when the hammer came down, with the overall sale bringing around ten times the initial estimate.
It isn't only art that commands high prices when associated with celebrities.
James Brown's sunglasses sold for $8,750 in 2008, while Marilyn Monroe's bathrobe sold for $120,000 in July 2009.
Even a piece of Britney Spears's chewed bubblegum sold for $14,000 in 2004.
"Celebrity memorabilia are all about passions" said a spokesman for a London auction house. "If you get two die-hard fans into a bidding war, the final price could be anyone's guess."