Tunisian-born and Sorbonne-educated Sami Hassoumi, a one-time resident of Paris, specializes in $2 million-and-up luxury townhouses, condominiums and co-ops. With Brown Harris Stevens since February 1996, he is one of the firm's top producers. His townhouse sales, mainly to live-in owners and investors from overseas, have been some of the more outstanding examples in Manhattan, including 31 Beekman Place, 15 East 88th Street, 7 East 67th Street, 17 East 75th Street and 161 East 64th Street. He is also very active in the coop/condominium markets having most recently had co-op sales at 870 Fifth Avenue and 535, 563, 625 and a penthouse at 888 Park Avenue. Among the condominiums he has sold are a nine-room property at 21 East 66th Street, nine rooms at 26 East 63rd Street, seven rooms at 188 East 64th Street and six rooms at 15 East 69th Street. Fluent in French, Italian and Arabic, Mr. Hassoumi loves horseback riding and commutes between his Manhattan home and the homes he maintains in