90 8th Avenue, one of the premier co-op buildings in Park Slope, has had two apartments change hands in the last month. On August 25th, Apartment 6B sold for $1,300,000; the 1,860-square-foot three-bedroom originally was listed for $1,395,000 in June and was in contract by the end of July. A week later, Apartment 2A, a two-bedroom that was Co-op of the Day back in February, closed for $725,000, 12 percent less than its original asking price. GMAP