LONDON—With auction houses no longer offering much in the way of guarantees, art sellers are less confident about putting their property up for bid, leading to estimates at this month's London sales that are 70 percent lower than they were last year. The auction houses announced at the end of last year that they would no longer be offering minimum prices to sellers after several high-value guaranteed works failed in 2008's last quarter.

At evening Impressionist/modern and contemporary sales by Sotheby’s, Christie’s, and Phillips de Pury & Co. in the British capital, the number of high-value lots by such major-name artists as Picasso, Warhol, Bacon, and Prince has dropped. And while last year six guaranteed works sold for more than €10 million ($16 million), no piece is expected to fetch more than €10 million this time. Without guarantees, “people are trying to negotiate private sales,” says Sotheby’s head of Impressionist auctions. “They’re saying, ‘This is what I want. If you can’t get it, I’d rather not sell.’ ” London auction week begins next Tuesday, June 23.

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