The seller says the stair is a "collectible" and could be used as a coffee table if you get some cinder blocks. He also states that "I know for a fact that only three of them exist outside of the retail stores' circulation, and this is one of them." According to the seller the stair originally cost $10,000 from a German manufacturer. That manufacturer would be Seele GmbH & Co. For a complete history of the Apple retail staircases, check out this ifoAppleStore article.
Want it? Well, you're going to have to pick it up in Brooklyn, New York and the seller recommends you bring a car and a friend or two -- the step weighs 250 pounds (114 kg).
Thanks to Mark for the tip!