A blind person could tell the difference between the forged animation artwork being sold by Pittsburgh-area Gallery on Baum and the genuine article. Some of the forgeries on eBay, however, are more carefully produced and thus more difficult to distinguish from the original artwork.
One seller on eBay, Hkleiman, has been selling what appear to be forged pieces of animation art for at least the past year. [UPDATE – Sept. 29, 2011 : Since I originally wrote this post, I have spoken to both HKleiman and multiple other reputable people in the animation art community who know Hkleiman. It turns out that he has a long history in animation art and is a well respected seller. He has since amended some of his eBay descriptions and stopped selling these particular pieces in question. Although there are countless disreputable sellers on eBay, HKleiman is not among them and one of the good guys.]
They were first discovered by DisneyFakes.blogspot.com. The artist who runs the blog carefully analyzed the clean-up drawings with the finished film frames, and discovered numerous discrepancies consistent with forgeries. The artist, who asked us to keep him anonymous, knows a thing or two about clean-up since he worked as an inbetween clean-up artist on The Princess and the Frog.