NEW YORK CITY.-In its most aggressive move to improve their business, Sotheby's has announced an increase in its buyer's commission to 19.5 percent for items selling for up to $100,000. Previously, the commission was 20 percent on the first $15,000 and 15 percent on sales between $15,000 and $100,000. Christie's rates remain the same. But in a setback for Sotheby's, on Monday, Moody's Investors Service cut its ratings for Sotheby's bonds, which are already at very low levels. Moody's noted that though Sotheby's had $107 million in cash as of December, the company may not generate enough operating cash to run its business and meet bank payments over the next 12 to 18 months. A surprise decision by a U.S. appellate court on March 13 has also damaged Sotheby's prospects by raising the possibility that the company may have additional liabilities to foreign customers in its antitrust case.