Complex sentence: When the apartments were first offered for sale in 1991, they were the highest-priced residential apartments ever offered for sale in New York City; their sale prices set city records in 1993 and 1994.
Split into simpler sentences: When the apartments were first offered for sale in 1991, they were the highest-priced residential apartments ever offered for sale in New York City. Their sale prices set city records in 1993 and 1994.