A company led by Walter DeFortuna, the chairman of condo developer and brokerage Fortune International Realty, paid $18 million for a pair of apartment buildings in wealthy Bal Harbour. Bal-Harbour Quarzo, managed by Juan G. Arcila, sold the 31-unit building at 10250 Collins Ave. and the 36-unit building at 291 Bal Bay Drive to Beach Haus Bal Harbour, managed by DeFortuna.