This piece in The New York Times is primarily about jewelry, but also includes the following:
"After the spectacular case in 2000 when a fake Richard Diebenkorn painting was nearly sold for $135,000 on eBay, the company put in place a handful of safeguards, like the PayPal buyer protection plan, an improved system for spotting eBay policy violations, and improved detection of fraud in general. But when it comes to counterfeit goods, the problem has gotten worse.
"Artwork is particularly vulnerable to counterfeiting. 'The majority of things that appear on eBay are fakes,' said Joel Garzoli, an art gallery owner in San Rafael, Calif."
UPDATE: Professors Bainbridge and Ribstein comment on the business and IP aspects of the story (and continue the debate in the latter's comment section).