ArtFact.com: Auction site for art collections. Subscription may be needed to obtain some information.
Artinfo.com: art prices and appraisals.
Heritage Auctions: keeps an archive of previous auction results with high-res scans, including film material and posters.
eMoviePoster.com: Commercial auction site with extensive archive of past results in high resolution (although watermarked).
Poster Auctions International.
Royal Athena Galleries. Auction site with images of past sales objects. "The world's most extensive selection of fine art of the ancient world."