Sotheby's, departing from its role as an auction house, is financing the $142.8 million purchase of the contents of a distinguished New York art gallery. Sotheby's has agreed to finance the purchase of the Pierre Matisse Gallery, one of the foremost inventories of 20th-century art, in an agreement with art dealer William Acquavella, said Diana C. Brooks, president of Sotheby's in North America. The gallery was founded by Pierre Matisse, the son of painter Henri Matisse. Its inventory has 2,300 works by artists such as Joan Miro, Marc Chagall and Alberto Giacometti. Acquavella will be managing partner of the joint venture company, which will be called Acquavella Modern Art, The New York Times reported today. Acquavella said he plans to sell the paintings privately over a period of time. Until now, Sotheby's has represented sellers and has not owned most of the works it has sold. But with this move, Sotheby's acquires a substantial inventory of artwork and enters the gallery business. Sotheby's has expanded its operations in the art world since real estate tycoon A. Alfred Taubman bought it in 1983. The auction house in 1988 started Sotheby's Financial Services, a subsidiary that lends money to sellers, buyers and owners of art. More recently Sotheby's has offered guaranteed minimum prices for consigned pieces of art, even if the work does not sell at auction. ``I think once more it is another confirmation that Sotheby's has really become an art dealer,'' David P. Tunick, a New York art dealer and board member of the Art Dealers Association of America, told the Times. ``To any longer describe themselves as an objective middleman is wrong.''