But if Sotheby's is to remain competitive, it will have to lower its sellers' commissions also. Matthew Weigman, a spokesman for Sotheby's, said, ''we will be reviewing the changes.'' Mr. Dolman of Christie's said the new rates would apply only to the company's higher end or what he called its ''principal sales rooms,'' which allows it to retain the ability to compete with the online auction businesses like eBay that charge only 7.5 percent to sellers and nothing to buyers. The principal sales rooms include Christie's headquarters in London, on King Street; its Rockefeller Center location in New York; and its Los Angeles sales room. Christie's lower-priced auction rooms like South Kensington in London and Christie's East in New York will change their sellers' fees in the coming weeks, the company said. Its other European and Asian locations, including Amsterdam, Hong Kong and Singapore, will also change their fee structures but the rates have yet to be determined.