The new rule arrives as a means of protecting eBay from becoming embroiled in dubious legal matters. In essence, it's a reworking of an existing policy which states that all digitally delivered goods sold must be the intellectual property of the seller, unless said seller is specifically authorized as a distributor. When it comes to user-created content in games such as City of Heroes and World of Warcraft, the line between that which belongs to the publisher and that which belongs to the user is somewhat blurry.
Of course, eBay's implication that user creations belong to the developer or publisher is a topic that has seen many reams of internet debate. If Blizzard decided to auction off a special WoW item, would eBay allow it?