After a burst of sales in the first two quarters of 2017, Hamptons sales activity leveled off as listing inventory slipped in the third quarter. There were 517 sales, unchanged from the year-ago quarter but 12% higher than the 462- sale quarterly average of the past decade. Listing inventory continued to fall for the ninth consecutive quarter. This was in contrast to rising supply in the luxury market, representing the top 10%, as speculative development…