An aerial view of the grounds of Villa Firenze in Beverly Hills, California. Concierge Auctions When high-priced mega mansions for sale languish on the market — sprawling, flashy and unable to attract a buyer — some owners turn to a double-edged option: a real estate auction. The auction block can move trophy properties with nosebleed-worthy price tags quickly, after years of idling. But it comes with a harsh reality. An auction can act as a metaphorical real-estate guillotine, slicing inflated asking prices in half or worse, according to a CNBC review of recent sales. The three highest-priced homes ever to sell at auction each came up for sale in the…