Arguably Martin's finest.. the Age of Steam's cover art is now on sale at eBay. Part of a series of his games' art cover he's putting up on auction as part of a charity drive.
Current bid is already USD369.00 woooo....... If you are loaded and interested, bid here.
Here's the link to the Geeklist that's tracking all the auctions for his art cover.
http://www.boardgamegeek.com/geeklist.php3?action=view&listid=15057