Commerce Secretary Ronald H. Brown hopes to trade in his salesman's sample case for new pinstripes. Buoyed by his triumphant trade mission to China in late August, Brown is lobbying to replace Warren M. Christopher as Secretary of State. Brown tells confidants that with economics driving U.S. foreign policy now, he has proven himself to be a preeminent practitioner of "commercial diplomacy." Indeed, his success in prodding China to resume human-rights talks with the U.S. contrasts with Beijing's snub of Christopher's human-rights pleas on an earlier trip. Just in case he doesn't get the new job, Brown is planning a high-profile trade mission to India in January and a China trip in the spring.