First get the max sale date by complex and unit
Then get all the details you want.

select Buyer_Name, Buyer_Address
from table_name t
,(select complex_name, unit_number, max(sale_date)
from table_name
group by complex_name, unit_number) b
where a.complex_name = b.complex_name
and a.unit_number = b.unit_number
and a.sale_date = b.sale_date