After 75 years of family-run operation, Bob Slate, Stationer is seeking a buyer for its three stores in Cambridge—two in Harvard Square and one in Porter Square. Founded by Robert Slate in the 1930s and handed over to his sons Justin and Mallory over 30 years later, Bob Slate has been a notable presence in Harvard Square, selling an eclectic mix of paper, cards, pens, and other stationery items.