A new company formed by Kohlberg Kravis & Roberts has offered to buy 77% of Seaman Furniture Co., a large Northeastern furniture chain, for about $273 million. Members of the Seaman family have agreed to tender their 43% share of the furniture company to SFC, the two firms reported. Morton Seaman, president and chief executive of Seaman, will remain chief executive officer, they said. The new company, SFC Holdings, has offered to buy the stake in Carle Place, N.Y.-based Seaman at $26 a share, they said. KKR is a New York-based investment firm.