Millionaires down on their luck now have an exclusive place to sell their Italian castles, diamond chess sets, and super-yachts. An Internet auction site devoted to the super-wealthy launched officially Monday. BillionaireXchange said it has already played a role in the sale or exchange of more than $180 million in assets during a 10-month test phase. The website aims to exploit a market niche between internet sales sites such as eBay and auction houses such as Sotheby's and Christie's for fine art and collectibles.