Walk through the lakefront mansion that is going up for auction. Click here for a larger version.

A lakefront view and boathouse are among the many amenities of the mansion.

The seller of an 80-year-old Winnetka mansion has no idea what her lakefront estate might be worth in today’s market, so she is putting it up for auction. And several builders who want to unload their new houses or vacant land have signed on with a new online real-estate auction service. Both auctions are sources of potential bargains for buyers—although in the case of the Winnetka auction, it will be a bargain of the multimillion-dollar sort.

Set 100 yards back from Sheridan Road on a nearly two-acre lot, the Spanish Revival mansion—it stood in for the Cuban embassy in the 1987 movie The Untouchables—has a tile roof, tile floors, and arched windows and interior hallways. While the façade is quite formal, the rear of the mansion, which faces Lake Michigan, spills out through a terrace roofed with glass and carved wooden beams to a sizable pool, and from there down the lawn to the boat house and the beach. There is also a guesthouse, a tennis court, and gardens. Remnants of a shipwreck—perhaps part of the Lady Elgin, which went down in 1860—sit in about 15 feet of water some 100 yards from the beach.

“I can’t even estimate what this is all worth,” says David Kaufman, whose Schaumburg company, DK Realty Partners, is operating a sealed-bid auction for Carry Buck, who has owned the house for 30 years (she is the former wife of the developer John Buck). “There are no comparables, really,” Kaufman says. In the past few years, lakefront properties on the North Shore have gone for anything from $6.4 million up to $17 million, and each one is distinctive in its own way. “Every property is only worth what somebody will pay for it,” Kaufman says. “With these special properties, that’s going to be your answer.”

Kaufman urged Buck to go with a sealed-bid auction, in which potential buyers figure out what they think a property is worth. Each submits a bid without any knowledge of other bidders’ estimates, so it’s kind of a nail-biter for the bidders. All bids on the property are due October 7th. “[Carry Buck] might take the highest bid, or she might ask a few of the top bidders to participate in a live auction,” Kaufman says. (For more information, go to www.dkrealty.com/sbsale.html.)

Meanwhile, homebuyers in more down-to-earth price ranges might want to bookmark www.theauctionator.com. The month-old site, created by the entrepreneur and real-estate agent Christine Kempa, aims to be sort of the eBay of Chicago real estate, a place where homebuyers can bid online to buy a house or a piece of property. As with many other auctions, the Auctionator shows a “market price” for a property along with a minimum bid; the minimum is usually more than $100,000 below the market price. Not shown, however, is the seller’s reserve, or the minimum price the seller has said it must get for the house. Bidders will know they have hit that figure only if they are notified that the seller has accepted their offer.

Although Kempa’s company will also host live auctions (the next is this Saturday; check the site for details), the online auctions are more intriguing. If the company grows as planned, Kempa says, “we’ll have new properties going up for auction each day, and you’ll want to check back all the time to see what’s available”—which explains the Auctionator’s tagline: “You’ll Be Back!”

The site’s inventory is small now, with seven Chicago-area houses and about 20 buildable lots. Kempa hopes the site grows in popularity as builders in other parts of the Chicago area look for ways to get rid of finished homes inexpensively. For now, the listings are mostly limited to the far south and southwest suburbs, where Kempa sells real estate. There is also one listing for Skokie—and five for Nicaragua, which might count as the very far south suburbs.