Leading dealers have been approached by the auction house and offered incentives to supply art, antiques and jewellery for the revolutionary sales.
The British art and antique dealers were made an offer on commission if they agreed to work only with Sotheby's for the next two or three years. They have to sign within seven days.
The British Antique Dealers' Association has now written to its members, warning them to be cautious of the offer as other opportunities should arise as "virtual auctions" take off in the UK.
Sotheby's New York office is spearheading the company's global move into Internet auctions.
The announcement last week immediately boosted the business's share price and follows the success of other online auctioneers such as eBay in the States. Diana Brooks, president of Sotheby's Holdings, said: "It is clear that the Internet is superbly suited for holding auctions."
Some critics claim that if Sotheby's succeeds, they would have a virtual monopoly before rivals Christie's follow suit later in the year.
One antiques dealer said yesterday that the Sotheby's deal was "outrageous and very uncompetitive. There aren't enough of these goods for normal auctions and virtual auctions as well so they need to get the dealers on board. If they do, they know Christie's won't have a chance when they start."
As goods will not be examined by the auctioneers in the way they would be in a normal auction, it is understood that Sotheby's will rely on dealers with good reputations supplying items for sale. Most dealers sell on some old stock through auction houses.
Elaine Dean, of the British Antique Dealers' Association, said the proposition was an "interesting" one that they had to take seriously.
But she said: "We are urging our members to be cautious before they sign up for something for three years. We do feel that seven days isn't very long to sign quite an extensive contract. We're saying be careful."
An art trade insider said the Sotheby's offer seemed to have outraged many of the dealers. "They feel they are being pushed around. I don't think it has gone down too well."
David Redden, an executive vice-president of Sotheby's, said it was investing more than pounds 15m in the service and needed to ensure it had enough property to sell. The dealers he had spoken to were very excited by their offer.
Christopher Davidge, chief executive officer at Christie's, said details of their plans would be revealed on 1 March. But he said: "Christie's will not be proposing any arrangements that restrict competition in the marketplace, as we believe this runs counter to the philosophy of the Internet."