Usually, September is a great time to sell: nice weather, and everyone’s back from vacation. This year, judging by exclusive data provided by StreetEasy.com, it’s not so rosy. Research director Sofia Song looked at properties gone into contract in four Manhattan markets in September and October, and in almost every area, the numbers are weaker than last year’s. In the third week of September, the dip is 33 percent overall. Midterm jitters? Fear of a double dip? Just a hiccup? We’ll see soon whether we match those upswings last year in Week 43.