outside the Sotheby's New York headquarters Sotheby's sales growth accelerated in 2007 for the second year in a row on demand for contemporary art, enabling it to draw level with rival Christie's International. Sotheby's, which has its main salerooms in New York, said the value of art it sold in 2007 rose 51 percent, to about $6.2 billion. London-based Christie's said its sales rose 25 percent, to 3.1 billion pounds ($6.1 billion). Sotheby's stepped up its promises of minimum prices to sellers to win high-priced works last year. A Mark Rothko painting owned by David Rockefeller fetched $72.8 million in New York in May, a record for a postwar work. Guarantees for contemporary art in November were 78 percent of the low value of the sale, up from 35 percent a year earlier, according to an estimate by analyst Kristine Koerber at JMP Securities. Sotheby's overtook Christie's in sales after it sold Pablo Picasso's "Garcon a la Pipe" for $104.2 million in 2004, then fell behind the next year, based on auction totals. Sotheby's is incorporated in Wilmington, Delaware. Christie's is based in London. In 2006, Sotheby's sold $3.7 billion of art, up 36 percent from a year earlier. Auctions in 2005 were little changed. Sotheby's was at $30.69, down $1.16 at 12:37 p.m. in New York Stock Exchange composite trading.