 Tempur-Sili is the largest maker and seller of mattresses, and it wants to buy Mattress Firm, which has the biggest retail chain in the U.S., with about 2,300 stores. Anticipating that regulators would worry about the impact on rivals, Tempur-Sili said it preemptively signed agreements with six other mattress makers to keep their brands in Mattress Firm stores after the merger. But the Federal Trade Commission is unconvinced.